Be extremely careful if soldering Lithium-Ion batteries together, as getting them too hot (which isn't very hot compared to soldering!) can easily cause them to burst into flames or explode...
Now, I'll say, after 30-40 batteries, I've never had one catch fire. I've had a few that I probably broke as a result of heat, and I suspect that if you've tested the cells carefully and the batteries are in good condition, that they tolerate the brief heating better than cells that are already damaged, but that's not something I'm willing to bet my shed or my body on.
