Ive seen it done both ways, and in my experience both Brannon S. and Brendan M. are correct.
SaaS companies love to use signature date, as it offers more flexibility, but most other companies Ive worked for did it by payment date.
That said, Brendan -. is also correct; actioning commissions at time of payment is slightly harder, but you avoid clawbacks entirely. This is how it should be done (imo).
