Both replies are right on from a technical viewpoint. As a practical matter, I have been flying powered models since 1947 (control line in those days). I usually just put the propeller on as it comes from the store (of course, I smooth the sharp edges on molded propellers). I've never had a noticeable problem with vibration due to imbalance. When I first started flying R/C I would carefully balance the prop and noticed no practical difference. I do check a new prop for gross imbalance and have discarded some (mostly wooden props). A single cylinder, two stroke cycle engine has enough inherent vibration so that any additional due to a slightly out of balance propeller would be hard to detect without instrumentation.