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 $250.37   6.540%   $266.7442   $266.74   $266.75   $266.70  
  Purchase #2 $59.71   6.527%   $63.6073   $63.61   $63.60   $63.60  
  Purchase #3 $149.72   6.236%   $159.0565   $159.06   $159.05   $159.10  
  Purchase #4 $73.67   12.106%   $82.5885   $82.59   $82.60   $82.60  
  Purchase #5 $110.34   9.719%   $121.0639   $121.06   $121.05   $121.10  
  Purchase #6 $164.56   13.671%   $187.057   $187.06   $187.05   $187.10  
  Purchase #7 $32.72   6.721%   $34.9191   $34.92   $34.90   $34.90  
  7 purchase totals = $841.09       $915.0365   $915.04   $915.00   $915.10