From a structural point of view, what is holding up the orbiter on the launch pad prior to a launch? Are the tail service masts more than just a collection of conduits and plumbing?
Or, are the solid boosters supporting the whole stack?
Prior to launch the Orbiter was mounted on the External Tank and the External Tank was mounted to the Solid Rocket Boosters. The entire assemblage was referred to as "the stack".
The entire weight of the stack was supported by four posts at the bottom of each Solid Rocket Booster (eight total). The boosters were attached to the posts by large bolts (aka "studs"). At liftoff time the nuts on the bolts were exploded apart and the bolts retracted downward.
The tail service masts did not provide structural support to the stack.
Picture from 1982 Shuttle Press Reference Manual.
The only hold-downs are the eight SRB bolts.
This leads to the "Shuttle twang" when the main engines start. They're pushing to one side of the SRB hold downs, so they bend the stack just a bit. From "Space Shuttle Twang" by Tom Irvine in the Vibration Data newsletter (2010):
The orbiter’s SSMEs are offset from the vehicle stack’s center of gravity. The offset thrust from the Shuttle's three main engines causes the entire launch stack to pitch down about 2 meters at cockpit level, after the main engines start, but while the solid rocket boosters are still clamped to the pad.
This motion is called the "twang.” The boosters then flex back into their original shape due to internal stiffness forces. The launch stack pitches slowly back upright. This back-and-forth motion takes approximately six seconds.
At the point when the vehicle stack is perfectly vertical again, the hold-down post pyrotechnic nuts are ignited, the boosters ignite and the vehicle lifts off the pad. The twang is shown in the following video:
It's an engineering question as to why NASA didn't decide to hold the tail of Orbiter in place, rather than dealing with this motion. I haven't found that written down anywhere yet.
What this doesn't say is whether there's an static support before the twang starts: Is the shuttle sitting flat on something, without hold downs, just a surface to bear weight? This seems unlikely, because any over-twang would come back and hit any support pretty hard, but with the information I have I can't rule it out.