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 $80.45   10.487%   $88.8868   $88.89   $88.90   $88.90  
  Purchase #2 $100.96   7.104%   $108.1322   $108.13   $108.15   $108.10  
  Purchase #3 $136.72   3.690%   $141.765   $141.77   $141.75   $141.80  
  Purchase #4 $35.99   5.555%   $37.9892   $37.99   $38.00   $38.00  
  Purchase #5 $170.26   7.389%   $182.8405   $182.84   $182.85   $182.80  
  Purchase #6 $66.35   4.719%   $69.4811   $69.48   $69.50   $69.50  
  Purchase #7 $86.73   3.106%   $89.4238   $89.42   $89.40   $89.40  
  7 purchase totals = $677.46       $718.5186   $718.52   $718.55   $718.50