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 $249.58   5.859%   $264.2029   $264.20   $264.20   $264.20  
  Purchase #2 $112.36   5.682%   $118.7443   $118.74   $118.75   $118.70  
  Purchase #3 $16.63   9.162%   $18.1536   $18.15   $18.15   $18.20  
  Purchase #4 $136.63   10.713%   $151.2672   $151.27   $151.25   $151.30  
  Purchase #5 $52.66   13.330%   $59.6796   $59.68   $59.70   $59.70  
  Purchase #6 $172.74   8.652%   $187.6855   $187.69   $187.70   $187.70  
  Purchase #7 $140.56   13.114%   $158.993   $158.99   $159.00   $159.00  
  7 purchase totals = $881.16       $958.7261   $958.72   $958.75   $958.80