By all means, I tin the wire and the track. I actually just put a small blob of solder on the track and then just set the wire on top and apply heat. When it melts, the wire is securely soldered.
I have soldered to the bottom when I'm initially laying track, but truthfully most of the time I do the track drops after the fact. After you ballast, any solder joints are covered sufficiently in any case.
