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 $249.25   12.737%   $280.997   $281.00   $281.00   $281.00  
  Purchase #2 $11.84   3.567%   $12.2623   $12.26   $12.25   $12.30  
  Purchase #3 $131.71   6.198%   $139.8734   $139.87   $139.85   $139.90  
  Purchase #4 $47.37   10.264%   $52.2321   $52.23   $52.25   $52.20  
  Purchase #5 $58.44   5.866%   $61.8681   $61.87   $61.85   $61.90  
  Purchase #6 $155.49   6.401%   $165.4429   $165.44   $165.45   $165.40  
  Purchase #7 $64.86   9.119%   $70.7746   $70.77   $70.75   $70.80  
  7 purchase totals = $718.96       $783.4504   $783.44   $783.40   $783.50