Yes, typically you would have the account type as an attribute in the Account dimension.
The main consideration would be if other fact tables need to be associated with the account type directly. In that case it would be better to have a separate dimension. Think about a scenario where you have budgets that are maintained at the account type level and not the individual account level.
Also, if you find that you have the same attribute in multiple dimensions which could lead to confusion for end users...then it may be a good idea to consider a separate dimension. An example of this would be something like geography (country, state, etc.) which could be part of a customer, employee and store dimensions. In some cases it may make sense to split this into a separate role-playing dimension.
Hope this helps.