Hi David,
We still use 7.1.3 for our production server. Although I am using 7.2.1 on my machine, but I hardly have been developing OACS lately. There are still issues regarding 7.2.x look around the bboard. Here is a thread also that might help
https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=0005PD&topic_id=12&topic=OpenACS%204%2e0%20Design
I think... not sure OACS 4.5 will run out of the box with 7.2.1 since that is what happened on my machine. Although I haven't tested it very well.