It's not a matter of being necessary, but more the likes of being safe.
Chances are that the prop you buy will be perfectly fine. The plane you build may be fine too, but if not the following are just a few examples of the possible consequences:
- Your engine may wear out sooner
- Excessive vibration may cause your engine to come loose and possibly leave the airplane, striking some 10 year old kid in the head causing a trip to the hospital for him and a trip to court for you.
- Tail heavy plane is uncontrollably and scenario above happens

The decision is yours.

(sorry for being overly dramatic, but you get the point.)