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 $28.78   9.705%   $31.5731   $31.57   $31.55   $31.60  
  Purchase #2 $156.96   11.361%   $174.7922   $174.79   $174.80   $174.80  
  Purchase #3 $78.78   11.394%   $87.7562   $87.76   $87.75   $87.80  
  Purchase #4 $19.62   6.387%   $20.8731   $20.87   $20.85   $20.90  
  Purchase #5 $223.41   11.783%   $249.7344   $249.73   $249.75   $249.70  
  Purchase #6 $132.53   8.395%   $143.6559   $143.66   $143.65   $143.70  
  Purchase #7 $71.54   8.185%   $77.3955   $77.40   $77.40   $77.40  
  7 purchase totals = $711.62       $785.7804   $785.78   $785.75   $785.90