Use the grease caps to keep air and water from starting corrosion which would eventually spoil your shrink wrap or tape and eat through the wires or spread open solder joints too.

If the air and water can eat away at the heat shrunk insulation, then it should be able to eat through the insulation in the rest of the wire. The greese cap doesn't protect all of the wire.

What happens when you get the very end of your shoelace wet?
Does just the very end stay wet? No, capillary action can cause the water to wick into the stranded wire of the solenoid.

You can solder and wrap them or just use plain old wire nuts. I see that all the time. They can last for years before you have a problem. Just a question of what is your time worth, and why spend time doing something half right, when for .40 you can do it completely right.

There are some wire nuts with silicon sealant inside of it. I've used them rather than the hassle of a seperate wire nut AND grease cap.