They always owe me money rather than the other way around
If you know they're going to owe you money, I'd be doing it on 6th April last year.
This is one of those things where I really don't understand why so many people leave it so long. I know it's not nice (but really is very easy if your records are in order) and you might have to pay them money (btw, is everyone aware that you can do your tax return any time from 6 April and not have to pay in full until the end of January - I know that some people have to do payments on account, but many don't) but leaving it until the last minute when you've had nearly ten months to do it is madness.
The system could fall over, there could be a problem with your log in, your accountant is very busy, you could have information missing, etc etc.
I do DPs and he has various sources of income and some is PAYE, some is untaxed, and some is taxed at too high a rate (CIS) so it depends how much of what sort of work he has done whether he owes them or they owe him.
If he's done a lot of CIS, it'll be done in April, if he's done a lot of the untaxed freelance stuff, I might leave it a bit longer, but always do it over the Christmas holidays at the latest.