Double clutching is useful whenever you move the stick on an unsyncrhronized ****** (like your stock A Model).
If you DC going up, and you have sufficient 600 weight ****** oil, the DC will allow the gear cluster to slow down while the engine speed also drops in preparation for re-engaging the 3rd gear. This may NOT be necessay when the ****** is cold, as the gear cluster is slowed by the gelatinous mass that is the fluid, but as the temp comes up and the fluid thins, DC'ing will save you the gnashing of teeth (yours and the trannys).
Always DDC (double-de-clutch) coming down, until you realize that brake pads are much less expensive than clutches and gears, and then you might rely more on your brakes than the engine to slow the car.
The only DDC I do is when making a right turn in 3rd gear. De Clutch, let the car slow for the turn out of gear AND WITH THE CLUCH OUT, make the turn, and half way through rev the engine if necessary to synch the cluster to the road speed. Then clutch in and grab 2nd. If the car bucks, you need more revs, if it lugs, you slowed too much for the turn.
Some folks don't DC/DDC, they just wait a while longer before making the shift. This also allows the gear cluster to slow down to better match the slower running engine. Thick enough fluid is also key here, and a little STP with the 600 Wt is useful.
I can tell when my ****** fluid is down by even 1 ounce because the gears will want to clash even on an upshift with my normal DC. I just wait an extra second to allow the gear cluster to slow further, then engage the gear, then I recheck my fluid level, and yep-it is a little low.