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 $19.61   8.312%   $21.24   $21.24   $21.25   $21.20  
  Purchase #2 $31.41   6.566%   $33.4724   $33.47   $33.45   $33.50  
  Purchase #3 $143.48   13.846%   $163.3462   $163.35   $163.35   $163.30  
  Purchase #4 $72.46   7.132%   $77.6278   $77.63   $77.65   $77.60  
  Purchase #5 $128.54   4.708%   $134.5917   $134.59   $134.60   $134.60  
  Purchase #6 $136.43   8.674%   $148.2639   $148.26   $148.25   $148.30  
  Purchase #7 $47.83   11.630%   $53.3926   $53.39   $53.40   $53.40  
  7 purchase totals = $579.76       $631.9346   $631.93   $631.95   $631.90