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 $35.61   12.288%   $39.9858   $39.99   $40.00   $40.00  
  Purchase #2 $14.63   7.531%   $15.7318   $15.73   $15.75   $15.70  
  Purchase #3 $226.74   6.150%   $240.6845   $240.68   $240.70   $240.70  
  Purchase #4 $126.74   6.631%   $135.1441   $135.14   $135.15   $135.10  
  Purchase #5 $159.75   13.414%   $181.1789   $181.18   $181.20   $181.20  
  Purchase #6 $93.65   8.668%   $101.7676   $101.77   $101.75   $101.80  
  Purchase #7 $51.44   11.461%   $57.3355   $57.34   $57.35   $57.30  
  7 purchase totals = $708.56       $771.8282   $771.83   $771.90   $771.80