Generally speaking, parachutes were mandatory for all bomber crewman during a bombing mission. Parachutes were considered property of the USAAF, and individual crewman had to check out and check in their parachutes after each mission. This procedure applied to all crewman.

Crew members usually strapped into their parachute harnesses after take off, sometime during the rendezvous point at which the bombers began flying in formation. Of course, this was a general procedure, and sometimes crewman would forgo wearing their parachute as they thought it hindered their movement throughout the aircraft. Even though some crewman refused to wear their parachutes, they often kept them within arms reach, just in case.

The only exception that I am aware of is for ball turret gunners in B17's, due to their unique circumstances. To minimize any unnecessary casualties, the standard operating procedure for the ball turret gunner was to occupy the ball turret when the B17 crossed the English Channel (early in the war) or halfway into France during the latter years of the war. This was due to a couple of reasons: first, if a plane had engine or hydraulic malfunctions and were forced to turn back to the airfield, it could be disastrous for the plane to attempt a landing with the gunner still in the turret. Also, it was deemed unnecessary for the guns to be manned when in friendly territory, so the ball turret gunner was an extra crewman who could be used for other tasks. However, once the ball turret gunner occupied his position, there simply wasn't enough room for the gunner to have his parachute in the turret with him. Thus, his parachute was located right next to the hatch, and in the case of an emergency, the ball turret gunner had to rely on a crew mate unlocking his hatch and assisting him in getting out of the turret, after which he could grab his parachute and bail out.