Parallel charging of lipo batteries, maximum voltage difference?
Parallel charging LiPo packs has become very common in the RC hobby. Granted, there is not a lot of empirical evidence about how good or bad this is. Only the fact is that a lot of people do this on a daily basis.

Personally I have been parallel charging 6s LiPo packs for 2 years now with good results. I have some budget packs that are over 100 cycles, so in that regard, I am happy with the life I got out of the pack.

My parallel charging routine was not very stringent. I likely never charged packs with over a 0.25V/cell difference during this time.  I think in the future I will be more careful about the voltage of the packs. My recommendation would be to stay under 0.1V/cell difference. 

The problem with parallel charging is that it is quiet easy to make a mistake, and connect packs of dissimilar voltages. So if you are going to do this, I would always double check your pack voltages before connecting.

A very good resource on parallel charging can be found on the Tjin Tech site. This is a very thorough examination IMO. If you scroll to the bottom it also addresses the potential surge currents when connecting the packs, and also includes experimental measurement of current that is released at the initial connection.