In my practice, sometimes the charges aren't shared evenly by the Parties, and sometimes the allocation even varies by billable item. Does the program allow for that?

Yes, a great deal of flexibility is built right in to the billing process.  You will set up the allocation for each billable item in a Case and each can be completely customized as to whether it is billed hourly, at a full-day or half-day fixed rate, at a flat fee, or is pro bono.  The default for each billable will be to split the cost evenly, pro rata, among the parties, but the percentage allocation among can be set independently for each billable.

Learn how to work with billables in Managing Billable Items and Adjusting a Party's Billing Account.