Should I reboot my phone regularly? After noticing that my Galaxy S3 sometimes grew slow after a week or so of not rebooting, I set up a Llama task which reboots my phone automatically every night (under certain conditions).

My question is simply are there any downsides to rebooting a phone daily?
To answer your question, there is no downside if you reboot your phone on a regular basis. 

off topic: Android OS should not require a reboot, or it should not slow down your phone that much that you feel to reboot it. There may be a problem with an app or something, but that is for another topic.