Firstly, if possible it's is best to store scope with tube horizontal so it minimises dust. Secondly, don't stress about a little dust, its will have almost effect on the image. The big risk is if you attempt to clean it you can do real damage to the mirror.
It is a point of difference with some people never cleaning mirrors and others do it fairly regularly. If have only done it one in 18 months that I have had my 12" and it was a nervewracking experience!
There is a guide to cleaning mirrors here
http://www.iceinspace.com.au/63-345-0-0-1-0.html that Mike wrote. Personally it would have to be really dirty, with major thingys other than ordinary household dust before I washed mine again.
Malcolm