Unless you're changing to a lighter, more supple tire. I wouldn't think so.

I do it all the time on 700x32c Vittoria Randonneur Pros that actually measure 29mm when mounted on my Mavic Open Pros.

If they're true to size, they're OK for most gravel if it's not too coarse or deep.



Changes in tire size will make noticeable changes in handling. I swapped the stock, never ridden, 28c tires for some 37c's on my brand new Jamis Aurora tourer/commuter for the winter. I had to install KoolStop Thinlines to fit them between the brakes. With such big tires, the bike had a tendency towards hyperstability. It just doesn't respond quick enough to ride easily with no hands. However, if I do any loaded touring this year that's the sort of handling I want so I'll keep them around and may put them back on for next winter.

This year I put the original tires back on while screwing around last month. With the bottom bracket lowered by almost a centimeter, the bike handled much quicker although I didn't try riding with no hands through the foothills surrounding my house. The same thing happens when I put 700x25c Michelin Krylion Carbons on the Soma Double Cross cyclocross/commuter. I then tried the 700x32c Randonneur Pros that were pulled off when cross season started. I liked this last set of tires best even though they probably weren't the fastest.

For this reason, I'd hesitate to jump from 38c all the way down to 28c. Every geometry will have a corresponding tire size that will handle closest to your liking. Maybe go for some faster 35's or 32's if you're fairly certain the bike still will handle well. I think I'll be trying some 700x30c Marathon Racers, which supposely measure about the same as the 32c Randonneur Pros, on the Soma for the summer.