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 $44.67   12.464%   $50.2377   $50.24   $50.25   $50.20  
  Purchase #2 $228.48   6.535%   $243.4112   $243.41   $243.40   $243.40  
  Purchase #3 $126.28   4.861%   $132.4185   $132.42   $132.40   $132.40  
  Purchase #4 $217.32   10.423%   $239.9713   $239.97   $239.95   $240.00  
  Purchase #5 $87.73   12.711%   $98.8814   $98.88   $98.90   $98.90  
  Purchase #6 $160.79   12.229%   $180.453   $180.45   $180.45   $180.50  
  Purchase #7 $12.87   13.398%   $14.5943   $14.59   $14.60   $14.60  
  7 purchase totals = $878.14       $959.9674   $959.96   $959.95   $960.00