Nikon D300 Complete Manual page 140

The biggest issue with using more intense JPEG compression
at the base ISO value appears to be a very slight increase in
contrast.
Contrast is always more difficult to remove from an image
than it is to add it. Thus, I would recommend sticking to
JPEG fine Optimal quality if possible. The reason to use the
other settings would be to save space at the expense of
potentially irremovable contrast buildup and some edge detail
confusion at high sharpening levels.
Now let's look at what happens at a higher ISO value (1600):
Thom Hogan's Complete Guide to the Nikon D300
edges will come back to
haunt us at higher ISOs.
JPEG fine Optimal
quality. File size is
5.4MB. The higher size
indicates we've got
noise building false
detail. Colors have lost a
tiny bit of punch due to
noise, and I'd rate the
really fine edges slightly
worse than any of the
ISO 200 samples.
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