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 $18.92   10.778%   $20.9592   $20.96   $20.95   $21.00  
  Purchase #2 $168.79   4.468%   $176.3315   $176.33   $176.35   $176.30  
  Purchase #3 $139.41   11.806%   $155.8687   $155.87   $155.85   $155.90  
  Purchase #4 $176.42   4.375%   $184.1384   $184.14   $184.15   $184.10  
  Purchase #5 $21.60   7.334%   $23.1841   $23.18   $23.20   $23.20  
  Purchase #6 $148.42   7.291%   $159.2413   $159.24   $159.25   $159.20  
  Purchase #7 $89.65   6.142%   $95.1563   $95.16   $95.15   $95.20  
  7 purchase totals = $763.21       $814.8795   $814.88   $814.90   $814.90