Nocked Up raises crossbow bolt damage by a lot. It is a Tier 3 Ranged skill that matters if you rely on the crossbow for quiet kills on Marauders and Freakers.
Yes for stealth players who use the crossbow. Skip it if the crossbow stays on the bike.
Thorn In Your Side, Thief In The Night and the Drifter Crossbow.
Use regular bolts on soft targets and keep special bolts for groups or tough enemies.