is there some rule against using goad?

[ChaozK]

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.

This.

Tp issues in dungeons usually dont arise on bosses so you dont need to goad there but your primary concern are big trash pulls where everyone is AOE'ing.
Also giving goad to them when they are already out of tp means its too late, since the tp refresh of goad is too small to counter the high tp costs of AOE's, its rather meant to slow down the act of actually running out, which means you can sort of give goad very early into the pull.

TL,DR: Give goad when there is a big trash pull to one of these guys: MNK/NIN/BRD/DRG/WAR

Also minor gripe since i have seen it being done by a lot of NINs, PLDs dont need goad for AOE since they use MP for flash. Give it to the other dps instead.