When re-wiring I use crimp and solder ring terminals. I cover the solder area and about 1/4" of the wire with heat shrink. There's nothing wrong with spades but by using ring terminals there's four less connections at the coils to be concerned with.
Yes, a clean wire properly crimped into a clean connector makes a "forever" air-tight connection, so solder is not needed. However, the slightest contamination (oil from fingers, etc.) make a connection that will eventually corrode. The crimp then solder connections are the same except that the solder prevents air from getting to the actual connection and therefore no corrosion. Also, you can see your work (not covered in plastic) so you know it was done right. I have seen crimp connectors (spade and ring) where the person was too close to the edge and crimped the plastic only or barely caught the metal - everything seems to work at first but eventually it doesn't and it is a bear to find. BTW, It is important to use rosin core solder and not overheat.
