Does it weaken or soften the springs at all?

Or should I disassemble it first?

Leave the head on and fill it with some ATF, then drive it around, clean and no harm done!

It's a spare sitting in my garage....and has been for a while now. Ok to tank or should I rebuild?

The thing that gets hurt from solvent soulutions is plastic and rubber pieces.....you can clean any metal material with it however, you need serious lubrication after ward.....I would soak the head in oil