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 $16.61   8.269%   $17.9835   $17.98   $18.00   $18.00  
  Purchase #2 $86.69   7.428%   $93.1293   $93.13   $93.15   $93.10  
  Purchase #3 $64.72   5.828%   $68.4919   $68.49   $68.50   $68.50  
  Purchase #4 $156.48   6.166%   $166.1286   $166.13   $166.15   $166.10  
  Purchase #5 $107.46   8.834%   $116.953   $116.95   $116.95   $117.00  
  Purchase #6 $40.63   7.558%   $43.7008   $43.70   $43.70   $43.70  
  Purchase #7 $64.35   3.107%   $66.3494   $66.35   $66.35   $66.30  
  7 purchase totals = $536.94       $572.7365   $572.73   $572.80   $572.70