Depends on the floor, and what you want to achieve by elevating the amp, and if you are using a microphone.

Any hard flooring will cause the sound to reflect and might create unwanted echo whereas a carpeted floor will absorb some of the tone. Think about how your voice carries and echoes in an empty room versus one that has contents. Same with musical instruments and amplifiers. 

If you are in a room with hard flooring you can try putting a towel or small rug / mat under the amp to check the effect on the sound. You can also tilt the amp by placing an object under the front of the amp, like a book or piece of wood.

If you are in a studio, height is irrelevant as a microphone or direct patching to the board is used, and there are generally baffles and other sound control measures used to make sure the sound goes where it should.

Putting an amp on a stand helps with projection as the sound travels further before bouncing off other objects, and gets the sound waves higher if aimed at an audience. The rest is experimentation to discover how different positions may or may not affect the sound.

Hope this answers you question. Cheers!

So true that repositioning your amp will drastically change your perception of the sound.  Guitar frequencies are projected in a very linear fashion and will sound alot harsher when aimed at your head. (warranting "off axis" mic placement for recording) I prefer the warmer sound of an amp lower than waist level and sometimes even aimed to  the left or right. Thats just me.