Good call. If there is any doubt, there is no doubt!
If you're doing the kind of flying which requires suction instruments (for VFR that's only really the DI) then you'd better make sure that they work. A DI will appear to work with low suction, however it will precess more easily causing you to wander off heading. It creeps up on you and you don't notice it until it's too late. If you're instrument flying it can get even more tricky and because people are taught to trust the instruments it's easy to follow that slowly toppling AI - trust me, I've done it!
However (there's always a caveat!) it gives you an opportunity to learn more and progress. Now that you know you can't trust the DI, how about trying the nav flight only using the compass. It's perfectly possible and has so many opportunities to learn more. For example:
- Compass errors (during climbing, descending, accelerating decelerating and turning)
- It's in an awkward place so you can't look at it so often. This is actually a blessing in disguise. Once you're on heading, can you maintain it without looking at the compass / DI? It should be easy. Just find a spot in the distance that's lined up with the rivets on the nose of the aircraft and fly to it. That'll increase your lookout, reduce your workload and make it all much more fun.
Now that you've had an opportunity to think about it, discuss it with your instructor and try a nav flight with the DI covered. It'll take you out of your comfort zone, but that's the way we learn.
Enjoy