What the guys above say is true, however I also believe that a dirty primary mirror just has to have a negative impact on what you're viewing, and when i'm doing high-res planetary imaging, I don't want light scatter or lack of contrast or decreased resolution because of a dirty mirror.
I clean my mirror fairly regularly (2-4 times a year if it's needed), and I think most people have an irrational fear that cleaning their mirror will scratch it - therefore they conclude that a dirty mirror is better than a scratched mirror. And that's true, but if you take the proper precautions then there's no reason you should scratch it, and there's no reason you shouldn't clean it if it needs it.
There's a thread here which discusses many mirror cleaning options and alternatives -
http://www.iceinspace.com.au/forum/s...ead.php?t=4496 - my post is #17 but there are also other options in there.
I also agree with above though, that you need to determine if it's dirty enough to *need* cleaning. I don't clean mine at the first sign of dust.. but after a very dewey session at Lostock over the weekend, my primary mirror had water condensing right on the mirror, and so now it's left many residue spots so it still looks like it's wet.
Yes you can still view through it, but I plan to clean it again before I do any imaging with it.