Many tests by the military and space industry has proven that the crimp is more reliable and better than a soldered connection. However, that is a good crimp done with the proper tooling. A poor crimp like one done with a pliers may be quite unreliable so; unless you have the skill and tools, you are probably better off soldering the connection. And, yes, LiPos come with about a 50% charge, if they fall below 3.9 volts per cell, they may be damaged to the point where they are not salvagable. The fact that they have quite a bit of stored energy should warn you to be very careful on attaching the connectors. Be very careful to not let the cells short out as the fireworks can be upsetting to say the least.