I'd say the only sessions that should be at 100% effort are your intervals.......there's nothing the matter with pushing your limits over short sessions, particularly if it's on a track where there's somewhere to collapse in a heap afterwards
Balance this out with your steady stuff and you'll find your training pace picks up naturally and your body will adapt to faster running. Too much fast work without allowing for adaptation is asking for trouble.
As long as you don't go full out the day before a race you'll be fine. Having said that, I often do a short interval of maybe 1000-1500m at race pace the day before a race just to get a "feel" for what I need to achieve the following day and I don't think it's ever done me any harm.
Tom20- I agree and all of my "faster than race pace" work is done in interval sessions. 200m/400m's at around 800m pace and 1000-1500m's at around 5K pace. Too much time at that pace is asking for injury but get the balance right and it works wonders!
Ran a new 5K PB of 16:51 last night (Age grading of 81%) on the back of this approach