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I guess you can do both (dark frame and in-camera noise reduction.
Interestingly reading a book on astro-photography last week (Rod Woz) it said if you are doing a 3 minute capture, spend the same amount of time doing darks (for each and every shot you do) plus flats and bias... I guess that is to push the equipment as far as it can go. The longer your shot - the more noise you are exposed to so the longer your darkframe should be to replicate exactly that same electronic function.
Bias I presume is adjustment for the CCDs over or under emphasis of Red, Blue and Green (more reading yet to do). I presume flats are to address off axis error (where stars in the centre of view are lovely little circles, but stars towards the edge are less focused football shapes (unless you own an expensive APO or true RC type tube.
All this assumes you will import your shots into MaximDL or CS Photoshop etc to process the raw information and darks / flats / bias.
Marc the other part of your question on achieving critical focus - and everyone says over do it, and adjust as the night progress and your gear is subject to thermal expansion or contraction and/or mirror flop. I focus on say Sirus - roughly thru the view finder, then snap 5 - 10 second exposure checking the focus until its as good as I can get. DSLR Focus can likely help this by checking and minimising the size of a designate star/s - meaning you have obtained optimal focus.
This done its onto the main game and longer duration astrophotography. I had my first play last night - scopes both aligned, focus good, bulb functioned worked - and I did 10 seconds, 30 seconds, 60 seconds, 2, 2 and 4 minute shots (guided) until yep - the clouds all rolled in!
Post what I got a bit later on tonight.
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