I use Brother printers and around every 12hrs they do some sort of maintenance 'thing', they click and clunk for about 20secs then go back to sleep.
the one in my son's room has been idle for 18mths (he's overseas now) but I had to use it about 2 weeks ago and it printed just fine.
so +1 with you have to spin them up every now and then to keep it all fresh.
I read somewhere once that the rate at which an ink cartridge dries out is about the same as if you actually were printing a page a day, or week, or some such figure.