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 $126.89   9.677%   $139.1691   $139.17   $139.15   $139.20  
  Purchase #2 $11.98   3.142%   $12.3564   $12.36   $12.35   $12.40  
  Purchase #3 $44.29   4.446%   $46.2591   $46.26   $46.25   $46.30  
  Purchase #4 $62.69   8.497%   $68.0168   $68.02   $68.00   $68.00  
  Purchase #5 $101.66   6.163%   $107.9253   $107.93   $107.95   $107.90  
  Purchase #6 $248.67   5.475%   $262.2847   $262.28   $262.30   $262.30  
  Purchase #7 $14.80   4.705%   $15.4963   $15.50   $15.50   $15.50  
  7 purchase totals = $610.98       $651.5077   $651.52   $651.50   $651.60