I guess it depends on the situation.
If it was a guy I loved, I tend to think we'd say I love you or something similar - e.g. I love it when you do this or that, that feels amazing, you're the only one who makes me feel that way, you're so beautiful, your skin feels so good etc - during sex so reassurance of his feelings/my feelings wouldn't be that necessary afterward. If we were getting up and carrying on with our day I suppose I would expect a cute little kiss and cuddle then for things to continue as usual. However if we were going to sleep again a kiss is nice and a compliment of some sort would go down well, you can't really go wrong. Maybe just chat about her day, ask her if she needs anything. That way she gets to talk and you can relax. If she hasn't learnt men like to go straight to sleep after sex and that its nothing personal maybe just say something along the lines of 'oh I'm thinking how lucky I am to be here with you' (she is probably fishing for compliments and reassurance). Then follow up with 'I'm worn out after that, you alright to go to sleep' - that way she knows you just want to sleep not avoid talking and any decent girl will probably just nod and curl up next to you :-)
I guess It would be nice to hear how much a guy loves me etc after sex but I think its nicer when its done out of the blue. My ex walked in on me doing the dishes once and I was dancing about to the radio and he just said you're so great I love you - the first time he'd said it. It just felt so genuine and unexpected it made me feel special. I don't tend to fish for guys emotions though or ask what they feel, I have many guy friends and live with 4 male housemates at uni now and I know they hate it. One has even said whenever a girl asks him what he is thinking instead of him thinking nothing or daydreaming about nothing as he was before she asked, his mind immediately goes to something dirty and he gets tongue tied lol!
As for the casual sex and one night stands etc, its a case of play it by ear. Most girls like different types of sex depending on the time of the month, sometimes we want lovey dovey comfort sex and at other times it can be very animalistic, as such the dirty talk etc should be adjusted accordingly, so try and interpret what your one night stand is after, ie. abit of comfort or a good roll in the hay. However with a one night stand it would be best to keep compliments casual, make her feel good but don't over do it, unless you want it to be more than a one night stand. Stick with you smell amazing, that feels fantastic don't stop, your hair is so soft, you have a nice smile. If you start saying she's beautiful and the best you've ever had she may get the wrong impression and want more than just sex - especially if you are just friends with benefits.
Hope that helps