I don't like soldering to the joiners, you're just adding one connection to go bad. Kinda' defeats the purpose of soldering in the first place. 
I've never had any issue soldering to Atlas track, and I don't remember ever melting any of the plastic ties. I like to come from the bottom, cut out the little piece of plastic between two ties, prep the surface using the Dremel to remove the anodize and rough it up, and just it it with the solder gun. The key when you're doing solid rail is additional heat for a much shorter time seems to be less damaging than trying to use a small iron and heating forever to get it hot enough to solder.
If the track is already down, then I just hit the bottom of the track and hook the wire there and solder it. Prep is key to doing it fast enough to avoid melting the ties.
