I would never ask my guy what i was out of 10 because i don't want him to lie lol.
If your relationship is trusting and genuine YOU should be able to ask, and HE shouldn't feel the need to lie.
Besides, a ranking of physical attractiveness against top model standards has little to do with how genuinely ATTRACTED you are to your partner. My guy is a good 50 kilos overweight. Nature has cursed me with a dick the size of a thumb. We still fancy each other
If your girl is asking that she just may need a bit of reassurance because she's feeling insecure at the time. But if a woman ask she should be prepared for the answer
Yep I think a lot of us guys don't get the insecurity bit (because we're not subjected to it to nearly the same degree) and fail to sympathise. But we also don't get the 'just fishing for compliments' bit. Like to us that's disingenuous.
'Ok if you just wanted to be told you look nice, just ask to be told you look nice instead of flying off the handle at me because I told you straight.'
Blokes really aren't into flattery or insincere comments out of politeness.
That said I think that directness has its merits which women often don't appreciate.
If, for example, a mate of mine is getting dressed up for the night and asks me with a hopeful expression if he's looking good in his newly-bought tuxedo, and he really DOESN'T due to his extremely skinny frame, I will feel compelled to tell him that he looks like a cross between a stork and an undertaker. This is to spare him embarrassment ; ie to prevent him going around preening in the mistaken belief he's God's gift to the female sex while the women can hardly contain their sniggers. I will then proceed to tell him what'll look best on him as far as I can say.
I value the same brutal honesty the other way round.