When treating my convertible top (fabric) with 303 High Tech Fabric Guard...should I be concerned about overspray getting on the rubber trim around windows & doors?  

No:  Though one would not immerse EPDM rubber in any petroleum distillate, even relatively mild Odorless Mineral Spirits (the carrier in 303 High Tech Fabric Guard) any overspray evaporates too quickly to cause harm.  However, to be doubly certain there are two additional things you can do:

1. After treating your fabric top, with a clean, dry, absorbent cloth wipe any overspray off rubber, glass and paint.

2. Get in the habit of using 303 Aerospace Protectant on the EPDM seals (door, window & trunk seals) on all your vehicles.  Not only does 303 Aerospace Protectant keep EPDM seals looking soft & flexible and looking like new, but 303 repels away potentially damaging chemical agents.