The watch seems pretty reliable. But, it is a programmable device which is subject to bugs. I'm not a WatchOS developer so I'm not willing to assert that a reboot won't cure some lingering or growing problem.
I thought the issue had gone away, but the other day I had to restart my watch since it didn't respond to vertical swipes. A restart always fixes that when it happens. I also seem to remember having some issue with Fantastical that required me to restart my watch to fix.
I feel the Apple Watch is closer to a computer than it is to a regular digital watch. I see from the replies in this thread that most people feel the opposite is true.
Having said all that, I don't feel it's necessary to restart or turn off the watch unless you are having some problem.