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 $78.61   6.208%   $83.4901   $83.49   $83.50   $83.50  
  Purchase #2 $60.27   13.435%   $68.3673   $68.37   $68.35   $68.40  
  Purchase #3 $147.96   11.176%   $164.496   $164.50   $164.50   $164.50  
  Purchase #4 $125.41   6.748%   $133.8727   $133.87   $133.85   $133.90  
  Purchase #5 $205.91   5.584%   $217.408   $217.41   $217.40   $217.40  
  Purchase #6 $250.67   11.323%   $279.0534   $279.05   $279.05   $279.10  
  Purchase #7 $55.96   12.704%   $63.0692   $63.07   $63.05   $63.10  
  7 purchase totals = $924.79       $1,009.7567   $1,009.76   $1,009.70   $1,009.90