In my case, I already have the SiriusXM and I haven't provided them with any CC info so if for some reason I see a Sirius charge somewhere, I will raise H E L L !
MB is pretty good with the free trials though, I just got a letter in the mail reminding me that MBrace is about to expire in 30 days and that I should call to cancel or I might get charged. With MBrace I did have to provide a CC to activate it but they are a completely different department from Sirius so i again I better not see any charges months from now.