It's my opinion that it would be easier with a background in music theory. When someone in these forums writes something really, really good and I find out s/he doesn't know anything about notation or theory, I always wonder how much more awesome s/he would be at writing if s/he understood music better.
I agree totally, and I can't see how knowing some theory can possibly be a disadvantage. I'd still be writing songs with very basic harmonies if I didn't know any theory. Having the keyboard has given me a chance to experiment in the way you describe - "what happens if I put such-and-such a chord here?"
Because I mainly write comedy I've even noticed that certain chords are better for comedy value than others. If I'm in C major, for instance, the note A flat sounds particularly funny for some reason. If I want to emphasize a comedy word then quite often I'll put A flat major under it, or F minor, or even D diminished. I've no idea why this should be but it seems to work. I'd never have guessed that without some knowledge of theory.
So, I agree with Tone that it would be good if you at least learned to play an instrument, like guitar or piano, at a basic level. Learning some theory would be a bonus.
Indeed. I don't think you need to be able to play
well - I certainly can't! - but it's very useful to at least know where all the notes are and how to put chords together. I'd love to be able to play the guitar but my fingers simply aren't nimble enough. Picking out chords on the keyboard is as good as it gets for me.
Of course, if you've got a particularly good musical imagination, it's possible to "hear" the chords in your head, which I can do to some extent. I've found it's particularly good practice to listen to recordings of songs that I know and transcribe the harmonies, then play them back to see how accurate I've been. (I'm quite often better than the chord transcriptions that you get on some internet sites!)