I think because greek is so very hard on the body, and too much can lead to serious issues, that a couple of things might be happening if she is saying that greek is "ymmv".
One is that she cannot handle another one that day, or possibly is not prepared for it. If you are looking for this service, advance notice appt might be required or recommended.
Two is size. If she considers you too large, for her, she would have to decline. It is better to say ymmv to give herself an out if someone is simply overly large.
If you are also asking, what's up with all the greek, then I think it is just trendy. An sp who provides it may do so because it is in demand, she can charge a premium for it, or because she knows most guys will not last nearly as long as they would in fs, so they are in and out faster and she gets extra $$. Very very few women actually get pleasure from it, and in some guys minds, that is the point, plus it is a very dominant position to be in -- getting off, believing there is pain suffered or at least little or no pleasure in it for her. Plus it is quite popular in porn, and as goes porn, so goes the imagination to fulfill lol
re: asking about something not on her menu. If her menu is not very detailed, and you are looking for something specific that is not listed, you could ask. If it is very detailed, then do not bother. If she does not have it there, you should assume she does not offer it, or at the very least is not going to provide it to someone she is not seeing regularly. It is not a good idea to ask about things during a session, if it is going to put her in an awkward position right in the middle of things. If you have a good time, want to return, and you are on your way to leaving, you can always ask then, just for your own information. I suggest waiting until after, because it is only afterwards that she will have a good impression of what you are like in a session, and that you are not being rude to ask.