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 $222.98   11.344%   $248.2749   $248.27   $248.25   $248.30  
  Purchase #2 $30.46   7.773%   $32.8277   $32.83   $32.85   $32.80  
  Purchase #3 $83.47   5.328%   $87.9173   $87.92   $87.90   $87.90  
  Purchase #4 $76.32   5.669%   $80.6466   $80.65   $80.65   $80.60  
  Purchase #5 $30.43   8.281%   $32.9499   $32.95   $32.95   $32.90  
  Purchase #6 $244.72   5.268%   $257.6118   $257.61   $257.60   $257.60  
  Purchase #7 $8.95   11.802%   $10.0063   $10.01   $10.00   $10.00  
  7 purchase totals = $697.33       $750.2345   $750.24   $750.20   $750.10