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 $247.26   12.757%   $278.803   $278.80   $278.80   $278.80  
  Purchase #2 $200.53   12.340%   $225.2754   $225.28   $225.30   $225.30  
  Purchase #3 $20.54   10.492%   $22.6951   $22.70   $22.70   $22.70  
  Purchase #4 $205.87   13.153%   $232.9481   $232.95   $232.95   $232.90  
  Purchase #5 $130.84   4.514%   $136.7461   $136.75   $136.75   $136.70  
  Purchase #6 $123.77   7.602%   $133.179   $133.18   $133.20   $133.20  
  Purchase #7 $29.61   9.158%   $32.3217   $32.32   $32.30   $32.30  
  7 purchase totals = $958.42       $1,061.9684   $1,061.98   $1,062.00   $1,061.90