I was told at an alignment shop that my bearings had slop, and the guy showed me that the back wheels had about an 1/8 of an inch of visible out-of-plane 'woggle' when jacked up and wiggled by hand. Just try that and see how much slop you get. Mine weren't making noise yet. I've seen bearings fail bad enough to WELD themselves to the hub spindle, so I don't think "wait until they howl" is the safest option.