I don't know how departing with the gear down helps if a brake is over the recommended temp, as opposed to over-temp (too hot). The temp limit in most cases is to ensure sufficient energy absorption in the event of a rejected take-off, so leaving the gear down after take-off won't help unless the brakes are over temp.
For my money, I wouldn't decline to taxi as I've seen brake temps drop 20 degrees or so during taxi on occasions.
Pugilistic Animus, you're probably correct. It'd be nice to have a definitive answer though.
In the Douglas Boeing 717 QRH, the relevant pages are entitled; Takeoff Brake Advisory Information, and, ESTIMATED Brake Cooling Time
From this I take the estimated cooling time chart to be a guide as to how long it should take for the brakes to cool to the required temp, not a mandated time to wait. If there's a temp gauge, surely the point is to use it.