Although I have never had any break on me in the cars I have owned (non toyota), when I did replace them I did see enough wear to justify the replacements. Some I changed early, others I changed late, but I always changed mine close to the mfg. specified intervals and always bought the OEM brand belts. I'm not saying you should get it done yesterday or your engine will die a horrible death. All I am saying is if I were you, I would plan on doing it at the next convenient maintenance opportunity you get. Also plan on doing some other inspections and replacements while you got the thing torn apart. You are in there anyhow so you might as well.