I have three WAN sites and cosidering implementing HSRP. Site A is connected to Site B (T1) and C (768K). Site B and C are connected to site D via fiber. I am considering implementing HSRP per interface on B and C because one of the vital equipment at site D requires a static IP default gateway. Never used HSRP before, is there anything I should be concerned about?
first of all HSRP is not a real routing protocol it provides a virtual default-gateway.
You can configure HSRP on D site routers and you can track the state of links between site D-B and site D-C on D site routers.
I would suggest you to have a dynamic routing protocol running on all sites and an horizontal link (can be a different subif/Vlan) on the D site routers so you can offer a default gateway to your equipment in site D and uses dynamic routing too.
In order to be able to communicate via HSRP if you use an ACL for antispoofing on site D client VLAN insert a line to permit destination ip 126.96.36.199 used by HSRP messages.
hope to help