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 $158.60   13.487%   $179.9904   $179.99   $180.00   $180.00  
  Purchase #2 $42.34   4.348%   $44.1809   $44.18   $44.20   $44.20  
  Purchase #3 $224.33   9.240%   $245.0581   $245.06   $245.05   $245.10  
  Purchase #4 $10.96   13.206%   $12.4074   $12.41   $12.40   $12.40  
  Purchase #5 $62.94   10.840%   $69.7627   $69.76   $69.75   $69.80  
  Purchase #6 $203.68   10.457%   $224.9788   $224.98   $225.00   $225.00  
  Purchase #7 $148.56   9.766%   $163.0684   $163.07   $163.05   $163.10  
  7 purchase totals = $851.41       $939.4467   $939.45   $939.45   $939.60