BEFORE can help set guidelines, strategize on where you want to go and how you want to get there...DURING probably good if you need to reference something such as a certain chord substitution, leading tone, or cadence etc. AFTER can be fun to just analyze your composition and see what you've implemented stylistically and technically.

May be best to just go without theory

Isn't this a case of it's always the best time? We should write down anything we hear in our heads, no matter how ignorant we may still be, and we should keep striving to expand our knowledge - it's a messy business of just pushing forward as best we can. So, I'll go with after, to vote for inspiration striking as early in our lives as possible!