I'd say that reading music is very important when writing songs, cause then you dont have to worry about figuring out what key your in and stuff.
If you read music, the key signature will tell you what key your in. If not, here's a litle trick that works most of the time, there ARE exceptions.
The first or last note or chord of the song usually tells you the letter name of the key your in, and wether it sounds happy or sad tells you if its major or minor. It still helps to understand a bit about music theory to apply this trick. It USUALLY works, there are exceptions like I said.