It's been a while since I've stayed at a Starwood - do they send an upgrade email after you've made your reservation? I recall that during the booking process they'll popup an "upgrade your room" option before I finish making the reservation, but I don't recall an upgrade offer after making the reservation.
Yes, all that I ever recall with SPG is time-of-booking popup vs. after-the-fact e-mail offer. The thing I've noticed is that the popup offer generally tracks to rates I was already shown but chose not to book (so I book a Jr. Suite @ $150/night, see that an Executive Suite is available for $200/night, then I'm offered to "upgrade" for $50/night to Exec).
It's just a guess on my part, but I'm thinking this has a negligible impact on elite upgrade availability - at least in the case of SPG's "reminder" approach.