Should I contact ATC if I don't have a transponder?
Every US controller I've ever heard from says they'd prefer to be talking to every aircraft in their airspace, period. This lets them know your intentions and allows them to move you around if needed, which saves them far more time than it costs giving you traffic advisories.

If you don't have a transponder, they can still tag your primary target with your tail number. Since you can't squawk or ident, they'll need a turn and/or an exact position for positive identification before they're allowed to do that. Once you're tagged, though, you look the same to them as someone with mode A only.

If you're practicing maneuvers, I suggest letting them know what to expect with something like "maneuvering at or below 3500 ft." That's a good idea even if you have mode C, but it's critical without it.