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 $14.66   10.556%   $16.2075   $16.21   $16.20   $16.20  
  Purchase #2 $50.68   12.302%   $56.9147   $56.91   $56.90   $56.90  
  Purchase #3 $38.30   12.827%   $43.2127   $43.21   $43.20   $43.20  
  Purchase #4 $78.64   13.199%   $89.0197   $89.02   $89.00   $89.00  
  Purchase #5 $162.91   13.858%   $185.4861   $185.49   $185.50   $185.50  
  Purchase #6 $239.38   13.588%   $271.907   $271.91   $271.90   $271.90  
  Purchase #7 $176.62   12.733%   $199.109   $199.11   $199.10   $199.10  
  7 purchase totals = $761.19       $861.8567   $861.86   $861.80   $861.80