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 $109.99   9.268%   $120.1839   $120.18   $120.20   $120.20  
  Purchase #2 $18.63   13.624%   $21.1682   $21.17   $21.15   $21.20  
  Purchase #3 $18.27   7.850%   $19.7042   $19.70   $19.70   $19.70  
  Purchase #4 $169.37   11.756%   $189.2811   $189.28   $189.30   $189.30  
  Purchase #5 $240.99   9.805%   $264.6191   $264.62   $264.60   $264.60  
  Purchase #6 $240.49   5.579%   $253.9069   $253.91   $253.90   $253.90  
  Purchase #7 $240.51   8.282%   $260.429   $260.43   $260.45   $260.40  
  7 purchase totals = $1,038.25       $1,129.2924   $1,129.29   $1,129.30   $1,129.30