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 $18.97   13.610%   $21.5518   $21.55   $21.55   $21.60  
  Purchase #2 $9.90   3.115%   $10.2084   $10.21   $10.20   $10.20  
  Purchase #3 $189.35   7.384%   $203.3316   $203.33   $203.35   $203.30  
  Purchase #4 $110.83   5.727%   $117.1772   $117.18   $117.20   $117.20  
  Purchase #5 $142.84   13.858%   $162.6348   $162.63   $162.65   $162.60  
  Purchase #6 $151.60   4.526%   $158.4614   $158.46   $158.45   $158.50  
  Purchase #7 $152.32   8.759%   $165.6617   $165.66   $165.65   $165.70  
  7 purchase totals = $775.81       $839.0269   $839.02   $839.05   $839.10