sip. in sip address or not?
 

I just signed up for a new account, where they give my SIP address as username @ provider.xx but Sipbroker's provider list has the domain as sip.provider.xx

Will either choice work, or will some providers enable both options, others only one? I think I've also seen some with sip0 sip1

It varies from one provider to the next. Most of them support both the Realm username@provider.xx and proxy username@sip.provider.xx addressing.