Tele,
I prefer the casters. They make it easy to adjust location and lock it in as needed. Casters may only be available for the older pyramid shaped cabinets that are wider towards the bottom. Not sure though.
I had a pair of orignal Walsh 2s for years that I put castors on myself for this purpose.
Good point regarding distance between speakers and center imaging. I think a good general rule is put them as far apart as possible while still maintaining a solid center image. Mono recordings can be useful to help determine this.
Also, in general, the farther out from the wall, the farther apart the speakers can be and still retain the solid center image.
If you loose the solid center image, the speakers are too far apart.
You may find that once the solid center image is locked in optimally for mono recordings, the stereo recordings then fall right into place with no further adjustments.
It took me over a year to finally land my Walsh 5S3s in their current location where they have resided now for about a year. My L shaped room is particularly problematic though I have found the omni OHMs to work quite optimally in there once I got everything right.
I have a really non-conventional asymetrical setup for my Walsh 2S3s in my small 12X12 office that I stumbled on, but that is a topic for another day.
One other thing I have found is that any changes or tweaks in the system otherwise generally results in some tweaks to speaker location being needed as well in order to optimize everything.
Good luck!