Currently client billing details must be entered individually for each site.  For clients which have multiple sites, this makes it very difficult to update credit card details. 

In my case I have one client with 4 sites, and another with 6 - so you can imagine the annoyance of 10 billing transactions and emails per month for 2 clients. 

PREFERRED BEHAVIOR

1. Support Client Billing Profiles, which I can then "attach" a site to when I am ready to bring it live.  This means that all billing history, credit card details, etc. are managed and updated in one place. 
2. When a new site is brought online, pro-rate the first month so that the billing cycle is synchronized for all of their sites, and they only get a single charge, and a single billing notification email per month.