Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $28.44   7.482%   $30.5679   $30.57   $30.55   $30.60  
  Purchase #2 $11.73   13.643%   $13.3303   $13.33   $13.35   $13.30  
  Purchase #3 $28.72   9.215%   $31.3665   $31.37   $31.35   $31.40  
  Purchase #4 $154.32   5.689%   $163.0993   $163.10   $163.10   $163.10  
  Purchase #5 $65.57   8.275%   $70.9959   $71.00   $71.00   $71.00  
  Purchase #6 $247.71   3.531%   $256.4566   $256.46   $256.45   $256.50  
  Purchase #7 $207.73   7.148%   $222.5785   $222.58   $222.60   $222.60  
  7 purchase totals = $744.22       $788.3950   $788.41   $788.40   $788.50