Ive found gettin the amp up off the ground does help, if the room you are in has a non carpeted floor. If you have a wood, concrete or tile floor, the sound is way too bright and echoey for my tastes. If you're in a carpeted room, I havent noticed much difference. Also, gettin your amp up off the floor will make it easier to mic, and you wont have to be on your hands and knees trying to find the sweet spot on the speaker.
Basically your questions answer comes down to personal taste in sound. Mess around with where your amp is in your room. If you like the sound you get when its on the floor, great. If you dont, try moving it. If it makes it better...go you.