It depends entirely on what is the dominant expectation for the characters involved.

Atticus, in the Ukiah Oregon books, has a canonical appreciation of women, but his long-term relationship with Ru means that if I were reading a fanfic and Atticus was paired with a woman, I would darn well want a het warning.

In NCIS, where Gibbs has been married four times, and dated several women during the course of the show, and where Tony has dated dozens of women, including one very seriously, and neither of them has ever talked about men, I expect some indication that they are going to be involved with men, however that indication is accomplished, whether through the romance genre tag, good story summary, or an explicit slash warning.

Hey, just had a thought here. These two men are also not dating their female co-workers in canon. If the author has them dating one of their female co-workers, I fully expect them to let me know that up front, too. So, I really do expect het warnings as well, they just usually come in the form of annoying mish-mashes of the names involved. Jiva, Tiva, Jibbs! I'd a whole lot rather just see HET! and the character's names listed in the Character A, Character B slots. Let's go on a campaign together to get people to list HET! when they put canons in male/female relationships. Then you'll see the word Het, and I won't see the stupid name mish-mashes in place of the word het.

On LJ, at least in most of the fic comms I subscribe to, it depends on the header.

Title: Example A
Genre: Angst
Characters: John Doe/Dr Smith

or

Title: Example B
Genre: Angst
Characters: John Doe, Dr Smith

That's my favourite sort of header. A slash denotes romantic involvement and a comma shows that both characters are present but not paired together. Some people miss out the genre line. Other people have a pairings line instead of a characters one, especially if it's a PWP.

I am mostly reading at the pit, and the warnings help me avoid these types of stories: "Characters A and B had been dating and making public displays of affection toward one another while at work in their jobs as officers in the US military.(even though, Character B doesn't even make public displays of affection in his het canonical relationships) And now our story begins." Or this type of story: "Yes we are 15,000 words into this story, and no, I haven't so much as mentioned that Characters A and B are in a relationship, but this entire chapter is a lemon that has nothing to do with, and was not foreshadowed by, the plot." (Actually, I would expect a warning for either slash or het in that case.) Or this one, and the one that I really like the warnings to be there to help me avoid, "Character A is a strong, mature, masculine guy. Because Characters A and B are dating, I am going to completely emasculate him and turn him into a simpering, needy shell of himself."

Really, now that I think about it more, I use the SLASH. DON'T LIKE--DON'T READ type warnings as part of the code that tells me this is most likely going to fall into one of those story types above that I want to avoid, and not some well thought out, well written, treatise on what canon failed to show us. I take them as a convenient Pit shorthand that indicates that the likelihood of this story being badly written is even higher than normal for the pit. Stories that are well written tend to have a summary that is well written and makes their relationship choices obvious, so they don't require a SLASH warning for people who don't want to read slash to be able to avoid the story.

As long as it continues to serve that function as pit shorthand for me, I am going to continue to appreciate its presence.