For me the main thing is to do a final check with someone sitting on the bike, so the chain's at (or near) its tightest (i.e. when the centres of the swing arm pivot, the gearbox sprocket and the rear wheel spindle are in line.) Under these conditions you want to see just a hint of slack, otherwise the bearings at either end are going to be overstressed, as well as putting excess load on the chain itself.
In my experience, dealers' workshops can be very bad at overtightening chains, so it's one thing I always check if I've had a service done by a dealer. The worst example I came across was a Tenere. It didn't come with a centre stand as standard, so the manual said to adjust on the sidestand. I'd fitted a (official Yamaha) centre stand, and the so-called technician (at a main Yamaha dealer) went and adjusted the chain while it was up on that. So once the bike was on its wheels and the suspension compressed a bit, the chain was like a bowstring, even without a rider. Fortunately I spotted it (though unfortunately not until after the guy had road tested the bike) and made a fuss, but I'm not going there again. It makes you wonder, if they can't get something as simple as chain adjustment right, how can you trust them to do something more major?
By the way, the rear wheel alignment marks on most bikes are often some way out. I check it with a simple pointer-type alignment tool that clamps on the rear sprocket to see that it's aligned to the chain run - not perfect, but better than relying on the marks. You can get fancy laser versions, which are presumably more accurate. It's easier than messing about with straight edges or bits of string, though it does mean taking off the chain guard. If the alignment is significantly out, you'll see polishing on the inner faces of the chain sideplates and/or on the sides of the rear sprocket teeth.
