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 $40.65   11.201%   $45.2032   $45.20   $45.20   $45.20  
  Purchase #2 $56.39   7.765%   $60.7687   $60.77   $60.75   $60.80  
  Purchase #3 $183.41   3.746%   $190.2805   $190.28   $190.30   $190.30  
  Purchase #4 $58.76   3.629%   $60.8924   $60.89   $60.90   $60.90  
  Purchase #5 $110.51   12.344%   $124.1514   $124.15   $124.15   $124.20  
  Purchase #6 $15.54   4.456%   $16.2325   $16.23   $16.25   $16.20  
  Purchase #7 $134.27   6.568%   $143.0889   $143.09   $143.10   $143.10  
  7 purchase totals = $599.53       $640.6176   $640.61   $640.65   $640.70