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

OP “…how tall should the stands be?…”

Good question… to bring the tweeters to your ear level in your listening position.

Blue Tack is the correct answer to your question (less is more when using it, IMO).

As far as the other comments go it depends upon the speakers you will be using.

For example "heavy" stands would most likely be a mismatch mated to speakers with lossy cabinet construction.

DeKay 

Solid stands are super important so they are not wobbly and loose allowing lost energy...not good. You want the speakers anchored to the stands with Blue tak or Posterboard putty ( this works really), and the stands you want no movement so each spike has to be adjusted (good stands have adjustable spikes or feet). In this way you will receive the full energy (which improves dynamics and Impact) and clarity of sound.

Matt M

OP, while you're at it, get a cheap laser measuring device.  Afyer a good height is found, get the distance from the speakers to your position exactly the same.  Use the laser along their sides to help point them the same, and vary toe-in until you get optimal imaging.  Vary distance apart.  This is how speakers "disappear".  Best $40 in audio I ever spent.

I do love my laser level, but not for speakers. All that 1/4 inch perfection seems silly when your head is not held in a clamp.    Eyeball height, toe to your preference.  

Experimented with all that spike and isolation stuff.  In deep carpets, spikes help stability. ZERO sonic difference.  My speakers sit on that waffle shelf liner on their stands. Won't slip and no resonances from the stands not being perfect. Not as messy as Blu-Tac.  (Vietnamese ceramic elephants) Spikes, balls, feet, just one more place where what may seem logical has been leveraged to make a lot of money selling snake oil with testimonials based on placebo effect. Want better sound? Buy better speakers. 

Funny how we buy bookshelf speakers and put them on stands.  I would have floor standers if my "domestic distortion" also known as my right to keep breathing or else, did not mandate the living room looks like a living room.  If it was a listening room, I would have full range floor standers.  Alas, the elephants stay.