I think it's better if you can play the piano but sometimes you also have to consider the instrumentations/orchestrations of your composition as each instruments has their own characteristics and sound. Piano makes it easier for you to play the melodies if you are limited to certain instruments but then again you can always ask other musician to check whether your composition is playable or not also there are music softwares that help you check the range of notes an instrument can play such as the Sibelius [that should make things a little bit easier].