Since they're so cheap, why not go ahead and replace them? If they start to fail in in year from now, you'll wish you had.
Alternately, you could wait until you 've got the motor out and give 'em a visual inspection. Mine were quite literally disintegrating; one of them came out in three pieces. Needless to say, I am replacing them.
I also found out today that Rustoleum's "Rust Reformer" is a pretty dern cool chemical.
'83 240D "Steiner"
'00 New Beetle TDI