Somewhat of an oxymoron - speaker stands for the floor for bookshelf speakers.

How much the sound varies with exact placement varies highly with the speakers. Ribbon and electrostatic are very sensitive to placement… as in 1/8th inch can make a significant difference. While this is a topic a different thread.. I think it is often possible to get great sound from ribbon speakers with a much lower percentage investment in the speakers… typical rule of thumb is 30%… for many years I had 10% in speakers (Apogee). But the amount of work with positioning and treatments (different than for dynamic speakers) was huge. To me that was the trade off. 

The Dynaudio Contour 20s come with a factory stand that bolts into the bottom of the speaker cabinet.  The stands are built very heavy, but I filled the open tube portion with sand to add more weight and stability.  They are hard to move now.

Same for B&Ws I use in AV set up.  I invested in the factory B&W stand; better build and weight.  Set at best height for optimal tweeter to seating position.  Again, filled open tube with sand.

Several aftermarket options are available, but worth investing in better ones with more stability and weight.

Different speakers are designed for different stand heights; while there is indeed a 'rule of thumb', there is no 'one size fits all' best speaker height for all standmounts. For instance, I've got a pair of Martens that are very high on their stands and ear level is slightly below the bottom of the woofer/mid, which is below the tweeter - that's how they're designed to be and they sound amazing. 

"Akgwhiz" has it right...final setup though.Getting it right pays off big.Sound anchors and done.Use his blue dots.They are pretty tacky ounce weight has been on long time.