I have built a couple of battery packs before, I used a 240 watt soldering gun to solder on tabs. Lots of heating power for a quick solder joint.
I dont think that heating the actual case or the lithuim in such a quick or small amount can actually hurt the battery.
What purposes the danger with soldering onto the cell is the way the lithium sheets and anode and cathode is connected.
Since the anode (-) is connected to the case of the battery the cathode (+) is probably the more dangerous end for over heating.
What connects the lithium sheets to the anode and cathode is a tin strip that looks to be spot welded to each.
If this tin strip comes lose from either end then the battery is dead, no connection from the lithium sheets to the outside.
If the tin stip comes lose from the cathode (positive end) then its hanging lose on the inside of the battery where it could touch the battery case which is negative and a direct internal short. 
HKJ has a few pictures that might help with understand what I mean. This is just my observation of how a lithium battery is designed.