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 $222.55   13.126%   $251.7619   $251.76   $251.75   $251.80  
  Purchase #2 $192.80   12.299%   $216.5125   $216.51   $216.50   $216.50  
  Purchase #3 $51.81   11.868%   $57.9588   $57.96   $57.95   $58.00  
  Purchase #4 $45.75   5.306%   $48.1775   $48.18   $48.20   $48.20  
  Purchase #5 $125.99   7.712%   $135.7063   $135.71   $135.70   $135.70  
  Purchase #6 $138.37   9.437%   $151.428   $151.43   $151.45   $151.40  
  Purchase #7 $90.74   5.747%   $95.9548   $95.95   $95.95   $96.00  
  7 purchase totals = $868.01       $957.4998   $957.50   $957.50   $957.60