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 $12.25   13.332%   $13.8832   $13.88   $13.90   $13.90  
  Purchase #2 $165.73   4.732%   $173.5723   $173.57   $173.55   $173.60  
  Purchase #3 $23.41   7.197%   $25.0948   $25.09   $25.10   $25.10  
  Purchase #4 $93.77   6.709%   $100.061   $100.06   $100.05   $100.10  
  Purchase #5 $15.51   4.175%   $16.1575   $16.16   $16.15   $16.20  
  Purchase #6 $186.39   7.454%   $200.2835   $200.28   $200.30   $200.30  
  Purchase #7 $20.37   8.776%   $22.1577   $22.16   $22.15   $22.20  
  7 purchase totals = $517.43       $551.2100   $551.20   $551.20   $551.40