The only issue with leaving things on an 'automatic' basis is that if something goes wrong - device failure, power outage, etc., there aren't a pair of human eyes to inspect things. Believe me, I could write a book about the horror stories...
Even if you put an ATO and auto-feeder, have somebody check on it at least every other day, and have somebody nearby, who is knowledgeable, on "standby" in case your friend/neighbour, gets in over their heads.
If you're having somebody manually feed the fish, get a 7-day pill box at the dollar store and put your portions in it (in the freezer if using frozen food). If you feed every other day - leave the non-food days empty so the caretaker knows. That way there is no chance of over-feeding unless the caretaker is completely incompetent.
We provide vacation service in our immediate area, but often people have a friend/relative/neighbour come to tend to their tank but they have our number, "just in case". Most people would rather pay an emergency service call to take care of a big problem, rather than have a helpless friend call or email them while they are away to tell them that something has gone kaput and everything died. And it happens... believe me it happens.
More often than not if an inexperienced caretaker calls us with a problem/issue, we can talk them through some simple troubleshooting over the phone (for free) and that takes care of things, but once in a while something big happens and we've helped a number of people whose tanks might have been in a world of hurt had somebody not been there to either take care of it or call in someone who could.
Not trying to freak you out or anything but Murphy's Law always seems to apply, and things decide to fail as soon as nobody is looking.
Jenn
