Bike Fit should be thought as a process; a proper bike fit will usually change over a cyclist's lifetime. As a rider matures and gains experience, position may change. Even after a 'fit session’, small adjustments may be needed. There are numerous bike fit schemes each having its own proponents. The 'ideal position' may not be possible at a given time. For instance, a rider may not have the flexibility to attain what would be the ideal position for him/her immediately.
The considerations in bike fit should include reducing risk of injury, creating a stable bike handling platform, ultimately a position that optimizes performance and comfort on the bike. For time trial bikes the comfort factor may decrease in importance in order to make gains in aerodynamics; since the time spent, except for long distance time trials, is relatively short compared to mass start road races or events and the handling demands are generally fewer, the position can be more 'aggressive' than on one's standard road bike.