is there some rule against using goad?

[MrCookTM]

On my Monk, I use Mantra. Maybe not all the time, but I use it

As for Goad, I just got it myself, and realized I can't see anyone's TP bar. So, if they don't ask, I can't give it to them, or I can guess, but then I could be wasting it for when someone does need it. Best to just use it when someone asks.

Not really. Goad slowly restores tp, it's not an instant refresh like invigorate. If you would see tp bars and use it once you see someone get's short on tp, it's almost too late. Goad is more about preventing to even get too low, or to prevent someone from having to use invigorate, so it's more about 'knowing' when to use it rather than 'seeing' it.
In raids this is relatively easy, since most groups have a set rotation for goad (i.e. first one goes to the monk 30 seconds into the fight, second one goes to bard as soon as it's up again), though this is not as effective anymore since nin tp nerfs (bard has to play paeon anyway)...
For other stuff like dungeons, i just give it to the tank whenever it's up, especially if it's a war.