I think it depends to a degree on your marker as well. Some can get away with being left dirty and some are complete prima-donas that require extreme care and attention. (THAT SAID).... Ultimately though, if you've spend hundreds of quids on a marker look after it! It is always good to clean off all paint, strip out the bolt clean the breach completely at a minimum. I always fit a brand new battery on the day of play. They are only 2 quid each and you can lose a whole game faffing around if your marker goes down. Then maybe once a year every six months depending on use, I would completely strip down and check everything. If you want to be lazy you can send it in for a service every year. But I can agree with Hobo_Joe getting to know your marker can save you a weekend of play.