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 $225.66   12.550%   $253.9803   $253.98   $254.00   $254.00  
  Purchase #2 $70.88   11.531%   $79.0532   $79.05   $79.05   $79.10  
  Purchase #3 $195.89   10.864%   $217.1715   $217.17   $217.15   $217.20  
  Purchase #4 $23.55   5.138%   $24.76   $24.76   $24.75   $24.80  
  Purchase #5 $129.43   10.756%   $143.3515   $143.35   $143.35   $143.40  
  Purchase #6 $187.65   9.211%   $204.9344   $204.93   $204.95   $204.90  
  Purchase #7 $30.31   3.507%   $31.373   $31.37   $31.35   $31.40  
  7 purchase totals = $863.37       $954.6239   $954.61   $954.60   $954.80