I'll do a lot of my own servicing (am changing a BB on the missus road bike this weekend for instance) but there are times when handing it over to a decent mechanic just seems the right thing to do - usually multiple things that need looking at. and especially MTB's where there are faffy jobs like suspension service, hydraulic brake bleeding etc. - they take up more time than it's worth so I'd rather pay for them to be done.
as for dirt - yes, give it a good wash first - they will appreciate that.