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 $209.40   10.388%   $231.1525   $231.15   $231.15   $231.20  
  Purchase #2 $170.31   3.564%   $176.3798   $176.38   $176.40   $176.40  
  Purchase #3 $198.81   4.544%   $207.8439   $207.84   $207.85   $207.80  
  Purchase #4 $7.55   12.866%   $8.5214   $8.52   $8.50   $8.50  
  Purchase #5 $62.46   9.377%   $68.3169   $68.32   $68.30   $68.30  
  Purchase #6 $33.61   10.565%   $37.1609   $37.16   $37.15   $37.20  
  Purchase #7 $83.68   11.626%   $93.4086   $93.41   $93.40   $93.40  
  7 purchase totals = $765.82       $822.7840   $822.78   $822.75   $822.80