Well I'm not an expert but the "Clockwork Recovery Mod" wiping process of the ROM (data, apps, battery stats, dalvik cache, etc) takes care of uninstalling your ROM.
Then you would "install" the new rom by flashing it using "ROM Manager".
P.S. After setting everything up, you could then use a file explorer app like found on fresh MIUI installs to delete any old folders/files of apps, music, video, pictures, etc found on your SD Card. Be careful and only delete what you know you installed on there and is leftover on your SD Card.
if by uninstall old roms you mean to delete the zip files of the old roms that you had before...then the answer is yes you can,cause you will have the nandroid backup(if you made one) and there's no need to keep the files unless you want to flash that rom again and if you want that you just copy the rom zip back to the root of your sd.short answer long:YES