They're hoping Geordi LaForge is in command - he ordered the only saucer separation where the intention was to safely preserve the civilians, and where the separation itself wasn't an untested dangerous high-warp maneuver. (The Arsenal of Freedom)

Riker, on the other hand, separated the saucer in order to shoot at a Borg cube from two directions at once. (The Best of Both Worlds)

Picard is the one who wouldn't drop out of warp to do the separation. (Encounter at Farpoint)

Riker also ordered saucer separation after a warp core breach was in progress and unstoppable. (Generations)

Those were the only four times the Enterprise-D was shown to separate.