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 $250.74   3.526%   $259.5811   $259.58   $259.60   $259.60  
  Purchase #2 $56.69   5.833%   $59.9967   $60.00   $60.00   $60.00  
  Purchase #3 $173.57   13.163%   $196.417   $196.42   $196.40   $196.40  
  Purchase #4 $226.41   11.401%   $252.223   $252.22   $252.20   $252.20  
  Purchase #5 $178.95   3.569%   $185.3367   $185.34   $185.35   $185.30  
  Purchase #6 $7.38   3.432%   $7.6333   $7.63   $7.65   $7.60  
  Purchase #7 $84.97   11.465%   $94.7118   $94.71   $94.70   $94.70  
  7 purchase totals = $978.71       $1,055.8996   $1,055.90   $1,055.90   $1,055.80