I wouldn't worry about it on modern plasmas, unless maybe if you do a large portion of your viewing in 4:3.
FWIW, my set is just about 3 years old and have been using black bars on 4:3 content since after the break-in period with no issues. If I have it on a 4:3 channel for most the day and turn on a 16:9 channel you can see the IR for maybe 2 minutes tops before it's gone.