I think that playing a string instrument isn't completely necessary, but it is very helpful, as is playing any other instrument. The important thing is to know what intruments can do, and how to write for them. If you don't have the time, money, etc. to play an instrument you should at least contact someone who does play the instrument, such as someone on this forum, or read about the instrument in a book, I don't actually have a name for a book, but I'm sure someone on this forum would know of one.
However, I would recommend playing a string instrument it is a great experience and is very fun, most of the time.