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 $17.38   8.636%   $18.8809   $18.88   $18.90   $18.90  
  Purchase #2 $140.72   7.510%   $151.2881   $151.29   $151.30   $151.30  
  Purchase #3 $9.57   13.406%   $10.853   $10.85   $10.85   $10.90  
  Purchase #4 $90.62   5.196%   $95.3286   $95.33   $95.35   $95.30  
  Purchase #5 $227.82   3.567%   $235.9463   $235.95   $235.95   $235.90  
  Purchase #6 $222.73   8.676%   $242.0541   $242.05   $242.05   $242.10  
  Purchase #7 $48.68   4.712%   $50.9738   $50.97   $50.95   $51.00  
  7 purchase totals = $757.52       $805.3248   $805.32   $805.35   $805.40