You may not. I have a small hole in one of my back molars too but me and the dentists agreed that a filling would be unnecessary as the whole causes me no pain as it is not deep enough to result in sensitivity issues.
Remember that in order to get a filling you need to drill into that hole and make it bigger. And the filling costs money. You will never have that hole clear up or get better if it has to be drilled into. The filling will need to be occasionally replaced. When the filling is not out-post drilling you may get sensitivity which you may not have atm as your pain is not so great as the hole is not so large.
So no, you may not need one. Just because you have a cavity that does not mean you have decay.
To answer your question - go see a dental professional, however, be warned, one dentist may give you a filling when another dentist would decide not to. Dentistry is not a clear cut thing. It isn't mathematics. And so who knows what is right without looking at your teeth? Nobody.