Square tubing works fine (my SB Cruiser trike is built entirely of it except the headtube, fork legs, and some pieces on the trike kit it grew out of), and IIRC may actually be stronger...but it is heavier for that strength than the round tubing would be (which is one reason round tubing is used). But I used it because it was easier to use and because I had a lot of it (and not much round).
If I were to rebuild the trike I would probably still use square tubing for most of it, simply for ease of assembly and ability to bolt flat panels to them. But if I were given the budget for it, I would not use the cheap mild steel I did the first time, but rather would use good chromoly tubing like from AircraftSpruce or similar places (out of my budget for now, though, at over a couple thousand dollars not including shipping).
On Crazybike2, I used round tubing originally, merging two bike frames to make the main frame, with pieces of other frames added as needed. But later I added square tubing for a number of other things (mostly because I had it), so it's something like 2/3 round and 1/3 square.
