Generally, my thoughts are:

1) Dungeons = DPS use it whenever possible.
2) ARR and HW Trials = DPS use it to kill last 5-15% of a boss.
3) SB Trials with Pugs = Save for Healer unless boss will die from a DPS LB.
4) SB Trials with team = DPS use it as soon as possible and hopefully get another LB1 or LB2 in sometimes.
5) Raids with pubs = Save for healer even if it goes unused.
6) Raids with team = see 4, unless still learning then see 5.