I've seen some varying tick rates used for RTS engines. Supreme Commander ran at a super-low 10 ticks per second. Is this normal for RTS engines, or do they normally have a higher tick rate?
Most RTSes I've worked on (or seen the source code to) simulate at 25-60hz. Slower than that, and characters start to feel unresponsive to commands and the controls feel generally laggy.
It's fairly common to run some game components only once per N ticks, however — ie, to undersample them. For example, run AI at only 5hz while the rest of the game is at 30hz, so that AI only runs one out of six frames. Or you can "think" only ¼ of the entities per frame to balance load, stuff like that.
I assume that you're using a fixed-timestamp and variable rendering framerate, which is the sensible way of doing it on all systems nowadays.
I can't see a big motivation for running > 20 ticks per second, because graphics rendering will interpolate anyway, doing more logic ticks just increases CPU usage without making the game better.
If you are going to be "clever" about collision detection, etc, you might manage with fewer ticks per second. Maybe 5?
There just need to be enough ticks per second that objects can't visibly move through each other (or other artifacts) due to the granularity of the simulation.
It's unlikely that a player's action will be responded to instantly anyway, the player expects some degree of lag when ordering objects to do things.
Construction, or even a simple "move this to there", is likely to take at least 5 seconds of real-time anyway.