The short, correct, catchall answer is, "it depends".
Each venue has somewhat different acoustics relating to whether to put bass cabs on the floor, or not. As a general rule, hollow floors, such as most stages, will have a bigger effect, which may be good or bad. And, generally, hard floors, like concrete, or tile on concrete, usually have less floor effect. Sometimes the floor effect is the least of the problems! Again, generally, an amp stand is the best way to get consistent stage tone.
But, there are exceptions to all generalities!! So, try different setups as much as you can, at least until you think you have a good feel for what works where. But even then, experimentation may surprise you, sometimes!