Most everybody seems to trim out stunters somewhere between 52 and 55 mph- 75-80 fps. At 88 fps you start to hear comments about flying really fast. Much over 55mph precision starts to go down because we have a pretty much fixed reaction time. Once you've got the model trimmed, as Randy pointed out, checking the lap time will stay very close, flight to flight, unless something changes. If it constantly changes, flight to flight, you've got problems to fix.
Igor also has a good point. Using the longest lines possible gives the plane more space so some maneuvers may present better(i.e. squares look squarer, even though the corner isn't all that tight), but a particular plane/engine combo may not do well on long lines. So part of the trimming process is finding the right length of lines for a particular plane. That is the reason for 48 oz. Noblers. On 60-62ft. lines it takes that much weight to keep .015 lines tight enough not to mess up the plane's trim. A lighter plane needs shorter lines and can fly a bit slower for good effect.