Will the placement of casters on speaker stands degrade the performance

Movement, Alternate Toe-In!!!

Vibrations.

Forget spikes. I’ve done it, my friends have done it, we all have done it.

The speaker frame/enclosure should be essentially vibration free. Some believe it is important to keep vibrations from the earth from getting ’up’ to the speakers (via spikes of course). A vibrating frame or enclosure will vibrate as much on spikes as off.

My speakers, on 3 wheels, are tilted back for time alignment and to aim the tweeters at seated ear height. The tops are tilted. See my System Photos, I have lot’s of Donna’s ’Stuff’ on top, (cork coasters below each). Nothing moves, except during the few steps I need to ’get back’ from the TT due to my springy floors.

Prior heavy Infinite Slope Model 2's came with 4 casters, I centered one rear one and added rear corner blocks to prevent tilting (just a bit shorter than the wheel height).
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Movement (marks needed for hard-found location(s),

Luckily my wood floor grid is centered l/r. I stuffed a speck of white paper in a joint for the inside front corners, then adjust toe-in maintaining that corner.

I would not live without being able to move my main system speakers. The weight of the speakers influences solutions: a slip sheet; self-adhesive felt pads; wheels. Solid, no rocking, but wobble or roll when desired.

My heavy speakers are on 3 wheels: 3 don’t need leveling like 4 might; more weight per wheel than 4; skirt prevents tipping. (the bottom of one speaker is shown in my System Photos here) (front block above front wheels provides the tilt).

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Specific Toe-In angles:

A single centered listener, both speakers aimed directly at me.

Alternate specific toe-in angle for two listeners, small table between, Wider center image needed for imaging. Aim the left speaker directly at the right listener. Air the right speaker at the left listener.

Alternate toe-in, album or track specific, i.e. Eurythmics, Blue Nile, Andreas Vollenweider, 3 Guitarists, anything ... too wide can be great, or not; too narrow can be congested: depends, use the toe-in to alter
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More Space Needed

Pushed back some when 1 leaf is in the dining room table, back into the corners when all 3 leaves are in the dining table.

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I call it Wide Center, actually it is L/C/R Imaging maintained over a Wider Area.

2 people listening to 2 channel, I alter the toe-in to get this.

3 people on the sofa (or 2 in the corners near their coffee warmers). my DBX Soundfield 100’s are specifically designed for ’Wide L/C/R Imaging’, great for Surround Sound. Center Signal is sent only to the Center Speaker, thus you NEED a dedicated center speaker, especially for dialog. Meanwhile, the Main L/R signals (info not present in the center signal/speaker) create the L/Phantom C/R imaging (good in only the middle, or maintained over a wider area?)

http://www.hifi-classic.net/review/dbx-soundfield-100-135.html

15 years ago I would have said yes.

Today I would say no. Trying to move a heavy object like a Magnepan 3.7 by sheer movement of the panels is impossible.

Unless they were stood on an a sheet of glass covered in oil maybe.

In fact the wheels could help lower frame resonance so they may even sound better.

Everything else is also a big plus.

Castors provide the ability to move heavy speakers easily.  I've used both spikes and castors- hard to notice any difference in sound quality.

Two ways casters harm sound quality. Main one is vibration control. Whatever the speakers are on has a huge effect on sound quality. Casters are rubber wheels on bearings, with some rake angle thrown in for good measure. None of this is good. All guaranteed to smear transients and lose fine detail. 

Second way is casters swivel around, making accurate speaker placement a lot harder, and leveling all but impossible. The speakers will never sound as good simply because they can never be in exactly the right place. 

I get why someone who has to move big heavy speakers around would be willing to put up with all of that, it is nothing compared to the hernia, ruptured disk or good old back pain you can get wrestling big speakers around. But you ask about sound quality, you get the sound quality answer: Put the casters on a platform, put Podiums on the platform, put the speakers on the Podiums.