As others have said it depends. Army and Air Force Guard/Reserve is about as close as you can come to guaranteed. Most services will have "guaranteed" aviation slots from time to time but it changes with supply and demand. Also, as others pointed out, the rules can change once you start. If a service suddenly finds itself overstaffed pilots those in training may suddenly find the standards for passing have changed. Those graduating may wear wings but find themselves flying desks or UAVs (as Mike likes to point out all roads eventually lead to the UAV).
I've even seen goofy situations. In the early 1990s just after Gulf War I the Navy was overstaffed pilots... alot. Pilots graduating from flight school were told they could get an unconditional release as there was no room in the Navy. Flash forward to the mid 1990s. The Navy was under strength pilots as they cut too many while the Army was over strength pilots (I knew an Army 1LT who graduated from flight school and was made the veterinarian clinic XO). So they worked a deal.
Pilots graduating from Army flight school were offered releases from the Army if they agreed to attend Navy flight school.
