LastPeriods Function

Does the LastPeriods function have to be used with the Aggregate function when creating a KPI expression?

Aggregate({[Incident].[Incident].&[1]} *

{LastPeriods(4, [Date].[Year - Quarter].[Quarter].&[2006-01-01T00:00:00])},

[Measures].[TOTAL])

If so, is it better to just do:

Aggregate({[Incident].[Incident].&[1]} *

{[Date].[Year - Quarter].[Quarter].&[2005-01-01T00:00:00] : [Date].[Year - Quarter].[Quarter].&[2006-01-01T00:00:00]},

[Measures].[TOTAL])

Whether Aggregate() should be used depends on the definition of the KPI - what are the business rules for it?

Also, {[Date].[Year - Quarter].[Quarter].&[2005-01-01T00:00:00] : [Date].[Year - Quarter].[Quarter].&[2006-01-01T00:00:00]} is probably equivalent to LastPeriods(5, [Date].[Year - Quarter].[Quarter].&[2006-01-01T00:00:00]) - so should 4 or 5 quarters be included?