I usually head towards the precelarance area as soon as the status of the flight changes to "Proceed to US Preclearance" on the board. Just to ensure I have enough time.
I'm flying to Tampa next month via Philadelphia and I can't help but notice that, despite preclearance in Dublin, the daily flights from DUB to PHL always seem to arrive at the international terminals (in PHL their international arrivals come in at terminal A west/east) rather than the domestic ones. I assume I can still just hop off the plane and scoot on past customs/immigration having already had that inspection.