Hmm thats a good question. If you made them common across all accounts, would allocations made in alternate accounts show up while viewing a different account? Like if I’m looking at checking and I had allocated $100 towards the mortgage from my moneymarket account, does that allocation show up in the totals shown on my checking page? Doing that would probably be best, but will make your category space much larger as all accounts will have to share the space if you have special categories that are specific to an account.
And here’s another pie in the sky suggestion if you go this route. You have category types – income,expenses(savings *nudgenudge*). make them collapsable in the category area. Allow the user to add custom category types and move a category between types if they want to. This allows user to by default have income and expenses, but by preference they could move some of their expenses to a different category type. property taxes that only get paid once a year. Self employment taxes that get paid 4 times a year. Business expenses vs personal expenses(for the self employed whose personal and business finances are more likely to be intertwined).
All these then being common and available on every account page, you could place the categories for a MoneyMarket account in its own category type(by preference, not enforced). This structure (assuming you can code it) should provide a highly flexible, easy to get started with, complete(with the savings category type) way for people to keep up with their accounts, cashflow, and specific savings goals.
Erm.. you didn’t have anything else to work on did you?