If you let them to produce flowers....the plant will start to 'run down' and the leaves fresh and vivid colours are not so bright anymore.
So yes, keep pinching and let the plant re-new its growth.
Coleus needs to be watered regularly...it likes moist soil condition...so decent size pot with good compost, enough moisture and pinching those flower buds off and there is no reason why your plant would not grow into good size specimen.