The instructions tell you to wipe down the cans before starting the bike, or the greasy fingerprints may burn into the outer muffler skin.
There is usually some smoking from inside as already mentioned, so I'd just wipe down the outside now if you are concerned, and not worry about it.
'10 Husaberg FE570
'10 BMW R1200GSA "30th anniversary edition"
'08 KTM 950SE-R
'08 Ducati Hypermotard 1100S