I noticed today my crank slipped a little, i tightened up one side and then other was fine.
Is it common for uni’s for bolts to slowly slack off and need tightening again?
What kind of cranks do you have?
I always check everything before I go out on a ride. I would suggest you do the same. My answer is: yes. Every few hard rides for me, I have to tighten my profiles, just a bit. They don’t come loose, but the bolt isn’t as tight as it’s supposed to be.
As far as I’ve seen with my unicycles, it’s a common occurence. Tighten them up imediately and don’t jump with a lose crank. It puts wear on the tapered hole in the crank and makes it skewed (at least, that’s what it did to mine)
zippy uses a KH unicycle with the splined cranks and hubs if that makes a diffrence
today, i had a hard fall right on my “boys” if you know what i mean…
i did a drop and the bolt holding my crank on was way loose and my crank came right off…
and it was because i didnt check and make sure everything was tight before i rode.
from now on im checking every time…
my KH is litterally off the shelf. I tightent the bolt, i don’t do jumps yet 
I noticed when i went to tighten 1 side didn’t want to and the other did, so i tightened it.
noticed again today the left crank slip a little - strange.