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 $9.43   8.342%   $10.2167   $10.22   $10.20   $10.20  
  Purchase #2 $13.39   13.186%   $15.1556   $15.16   $15.15   $15.20  
  Purchase #3 $43.75   8.439%   $47.4421   $47.44   $47.45   $47.40  
  Purchase #4 $118.68   7.765%   $127.8955   $127.90   $127.90   $127.90  
  Purchase #5 $91.77   13.469%   $104.1305   $104.13   $104.15   $104.10  
  Purchase #6 $245.59   13.451%   $278.6243   $278.62   $278.60   $278.60  
  Purchase #7 $90.72   7.125%   $97.1838   $97.18   $97.20   $97.20  
  7 purchase totals = $613.33       $680.6485   $680.65   $680.65   $680.60