[AngelicaDeAlquezar]

I agree with Rusty. It depends on what the speaker has in mind.
If you say "lo que no necesitas", you may be thinking of things the person wants right now, but doesn't need, so it's better to keep the money away.
If you say "lo que no necesites", you're then thinking rather of a possible whim (something the person isn't even thinking of yet) that might take away their money, if they don't keep it safe.