Try it, there’s a science behind smooth and accurate control systems. You might be ok with it’s performance. If you’re manually just speeding up and slowing down it will do what you input. Where PID excels is when you want smooth controls that takes feedback and self adjusts for load changes including hill climb ... etc without fluctuations we call oscillation. If you’re reading the rpm and having it adjust the speed based on a potentiometer you have already put in the “P”. This can get unbalanced if it starts to overshoot and hunt for the correction. Watch out for jerkiness.