Should I rotate the tires on a (grown-up) trike? I have a recumbent tadpole trike (though maybe this would apply to any style). Its two front wheels have a few degrees of inwards lean. The rear is of course perfectly vertical.

I've read in another question that for bicycles its not generally needed to rotate tires, and that any uneven wear may not matter much.

But on a trike the tires aren't changing their angle to the ground... and I think all three would normally wear differently.

Should I rotate them? Or reverse them? If so, how many miles would be a good rule of thumb?

Would the answer /advice be any different if there was no inward lean?
Swapping the tires around (or if they have some sort of quick release axles, the wheels) isn’t going to do any harm, and will prolong the useful life of the tire if they are contacting the ground at an angle. 

How often to do it is hard to tell. Depends on how many miles you ride / week, mass of you plus bike, roads surfaces etc. I’d probable swap tire around when I begin to see some wear.