Joe, all the manufacture says about ACETONE IS:
5)Cleaning after Soldering(1)Do not clean capacitors with the following cleaning agents:・Halogenated solvents: except for solvent resistant capacitor types, halogenated solvents can permeate the seal to corrode aluminum foilwithin capacitor.・Alkali solvents: could attack and dissolve aluminum can.・Petroleum based solvents: could deteriorate packing rubber.・Xylene: could deteriorate packing rubber.・Acetone: could blur print on sleeve.
When mixed with IPA @ 50% , it doesn't even do that. Some other cleaners do eat the rubber but not acetone. I wouldn't dunk them or soak them but an acid brush with some on there and a rinse with straight IPA won't hurt.
http://www.rubycon.co.jp/en/catalog/e_pdfs/aluminum/cautionalumi_eng.pdf