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 $8.82   6.639%   $9.4056   $9.41   $9.40   $9.40  
  Purchase #2 $12.60   5.196%   $13.2547   $13.25   $13.25   $13.30  
  Purchase #3 $18.56   10.185%   $20.4503   $20.45   $20.45   $20.50  
  Purchase #4 $166.42   7.160%   $178.3357   $178.34   $178.35   $178.30  
  Purchase #5 $134.90   3.808%   $140.037   $140.04   $140.05   $140.00  
  Purchase #6 $101.57   4.752%   $106.3966   $106.40   $106.40   $106.40  
  Purchase #7 $170.30   8.241%   $184.3344   $184.33   $184.35   $184.30  
  7 purchase totals = $613.17       $652.2143   $652.22   $652.25   $652.20