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 $18.67   8.438%   $20.2454   $20.25   $20.25   $20.20  
  Purchase #2 $200.62   4.596%   $209.8405   $209.84   $209.85   $209.80  
  Purchase #3 $165.92   10.664%   $183.6137   $183.61   $183.60   $183.60  
  Purchase #4 $225.61   5.871%   $238.8556   $238.86   $238.85   $238.90  
  Purchase #5 $222.35   5.710%   $235.0462   $235.05   $235.05   $235.00  
  Purchase #6 $250.71   10.113%   $276.0643   $276.06   $276.05   $276.10  
  Purchase #7 $230.69   10.663%   $255.2885   $255.29   $255.30   $255.30  
  7 purchase totals = $1,314.57       $1,418.9542   $1,418.96   $1,418.95   $1,418.90