There's no point loosening and re-tightening since it will a) make virtually no difference, and b) it would (supposing it made any difference) resettle very quickly, so by the time you're out of the scrutineering bay it will be illegal again. Sounds like cheating to me? It's unlikely the springs have sagged by much more than a few mm, and I can't see anyone running with more air in the tyres than would be their deemed optimum for any given condition.
If the shocks are the Bilstein ones (post '98 ISTR), then they are adjustable. The fronts are a bit tricky since there's no room between the coils of the spring for a traditional compressor, so a small ratchet strap (or 2) would be the best bet. Compress the spring, jack the car up and slide the bottom spring seat up. It sits on a circlip in one of 3 positions, and I would guess if you're under height then it's in the bottom or middle one. Prise the circlip out (carefully - you can't get them seperately!) and move it up. In the words of the great Mr. Haynes, refitting is the reversal of removal...
If they're not these shocks, then shims would be the only option I guess.
