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 $234.66   13.499%   $266.3368   $266.34   $266.35   $266.30  
  Purchase #2 $121.97   10.870%   $135.2281   $135.23   $135.25   $135.20  
  Purchase #3 $155.26   9.458%   $169.9445   $169.94   $169.95   $169.90  
  Purchase #4 $84.91   11.159%   $94.3851   $94.39   $94.40   $94.40  
  Purchase #5 $148.73   11.821%   $166.3114   $166.31   $166.30   $166.30  
  Purchase #6 $44.75   13.727%   $50.8928   $50.89   $50.90   $50.90  
  Purchase #7 $126.99   9.689%   $139.2941   $139.29   $139.30   $139.30  
  7 purchase totals = $917.27       $1,022.3928   $1,022.39   $1,022.45   $1,022.30