In my not at all humble opinion, if one is voting on a fiction category for the Hugos, one must read all the entries. If you don't want to do that, don't vote in that category. That's my basic rule, but I do recognize there's a continuum here.
Just Plain Wrong By My Standards:
"I haven't read any of the entries, but I like X author, so I'm going to vote for X author's entry." You have no business voting in this case. Please don't.
If you cannot be bothered to read even your favorite author's work before voting, don't vote. If you simply don't have time, okay, I get that shit happens, but again, please don't vote.
"I haven't seen any of the things up for Best Dramatic Presentation, Long Form, but I like the book that one of them was based on, so I'm going to vote for this."
I know someone who voted like that. I like this person, but please, please, do not do this! If you haven't seen any of the entries in the category -- if you haven't read them, if it's fiction (or non-fiction) -- you have no business voting on what the best item in the category is. You do not know what the best item in that category is.
Just Plan Wrong, But Officially Permitted
"I've only read one of them, and I liked it, so I'm going to vote for it."
"I've only read [any number up to one fewer than the entries], and I'm going to vote."
I hate this. I loathe it. But, it is officially sanctioned. I don't like that this is the case.
I want people voting who have actually bothered to read everything in the category. If you are eligible to vote for the Hugos, you are eligible to receive the electronic Hugo material as part of your membership to WorldCon. This makes reading everything in the category easier.
I do recognize that not everyone has a computer / e-reader / printer to print out all the stuff they don't want to read on a screen. I do recognize that time is a factor, and indeed, I may not vote on the novel category for exactly that reason.
I cannot and will not stop people from voting in a category where they've not read or viewed all of the material. I won't stop speaking to people who do this. But, I think it is wrong.
I Know It's Hard, But You Don't Get to Skip It
Oh, but Digger is sooo looong! The Martin novel is sooo long -- and I've not read anything in the series / I don't like this kind of stuff / I really don't want to read it.
Tough. If you don't want to read something that your peers have decided is worthy of a nomination, don't vote in that category. You can't make an informed judgement this way.
If it's part of a series that you haven't read, your informed vote is really important. You, more than any of the fans of the series, can judge how good that particular work is on its own, when it's all you have.
If it's part of a series that you haven't read, and you want to, and you don't have time to read the whole series, but you don't want to read just the part up for the Hugo, I sympathize, but please don't vote if you aren't willing to do the reading.
Now, on to what I consider grayer areas.
I Understand, But Resist the Temptation!
"I don't like X author's works."
It is entirely possible that you've given X author a fair shot. I would not know -- you would. And, I sympathize. I won't stop you from voting. I won't stop speaking to you. I can even respect this decision. But, I don't like it. Please, please try.
"I don't like this series."
I hear you. I share your pain. I've read the first Song of Ice and Fire book, and... okay, I didn't hate it. I did enjoy it for what it was, but I had and have no desire to read another one.
Nevertheless, that is exactly what I am going to do if I have any hope of voting on the Hugo for Best Novel this year. I am coming in after a gap of several books, but with a reasonable memory of the basics of the first. If I can't pick up enough to follow the book, this tells me something. And, it is something I'm in a better position to know than folks who've read all the books.
And if I run out of time to make an honest attempt, I will not vote on the novel category.
One of the pleasant discoveries I made last year was that Schlock Mercenary really is a good comic. I started with a 400+ (loooong) page book that was Chapter 11 in the series. I picked up enough to follow very quickly, and it was enough of a standalone to be complete in itself. It's hard sf, very well done. It read fast.
I Don't Like It, But... Yeah, Okay
"I tried it, but I couldn't finish it."
I don't like this. I'm not comfortable doing it. I may or may not do it this year, depending on whether I have time to read the two novels I have not yet read.
But, yes, while it may feel like fingernails scraping on a blackboard to me, if you give an honest try and bounce off a work, that's reasonable.
What is an honest try? Mileage varies, but for me, it does not mean thinking the book is merely mediocre. It might improve. It might not. If it doesn't, I need to know where it fits in the overall ranking, and for that, I do need to finish it. "I'd rather not finish it" does not equal "I can't read any more of this".
But, as long as you're giving it an honest try -- and you, not I, are the one who determines that -- go ahead and vote.