You once mentioned that April was conference season. What are the other months that would be bad for sending a requested manuscript to an agent the writer has a relationship with? And what is the calendar like for agents submitting work to editors? Obviously, December must be the worst month, but what else?
Let me clarify that it's conference season for me, but other agents might have different schedules. Traditionally I seem to be busiest March through July. I suspect that the reason is that earlier conferences contact me earlier, and because I only do so many each year I end up saying no to a lot of fall conferences. I find, though, that most conferences are either in the spring or fall, with very few falling in July or August.
If a manuscript is requested you should send it immediately no matter what month it is. When an agent requests something she's enthusiastic about it, and while you ideally want it to be as polished as possible, waiting can diminish her enthusiasm and your chances of impressing her.
As for good times to submit (and agents tend to work this way as well), I would suggest that the only time you don't submit is between Thanksgiving and New Year's Day. Because of the holidays and end-of-year crunch agents are rarely looking to pick up new projects and are instead cleaning house and preparing for the new year. Come January a lot of agents and editors are eager for a new year and new submissions. I know that writers talk all the time about summer being slow, but I have to admit, traditionally August has been one of our most successful months. Because summer is slow it gives editors (and agents) time to catch up on reading and find those gems they've been looking for.
Don't try to guess about the ideal time to submit. Just send it.