Personally I think I would prefer something like B,BBB.ss_sss_sss Examples:

Not sure why there needs to be a 'standard' as such. Any well designed UI will give the user options regarding how denominations are displayed. A comma is a traditional thou separator, both in maths and finance, for numbers > 100 I (for one) would not want numbers < 0 displayed with any separator.

The issue is that comma conflicts with decimal separator in many countries (especially in Europe, but many others). Apostrophe works better: 0.03'445'674 BTC