Hello John,
There is no more rebuilding of data mart is required as they release MR CU14. After upgrading to this patch rebuilding of DM is not required it's working as fine as Legacy option now. I just tested by adding new accounts and new budgets in GP 2015 and ran the MR report and perfectly showing the newly added accounts/budgets data without rebuilding DM. Just we need to run the check links of Budget table/account master table /setup table.
Thanks,
Habeeb