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 $232.81   11.652%   $259.937   $259.94   $259.95   $259.90  
  Purchase #2 $55.28   7.161%   $59.2386   $59.24   $59.25   $59.20  
  Purchase #3 $117.97   6.485%   $125.6204   $125.62   $125.60   $125.60  
  Purchase #4 $84.80   10.364%   $93.5887   $93.59   $93.60   $93.60  
  Purchase #5 $185.36   12.384%   $208.315   $208.32   $208.30   $208.30  
  Purchase #6 $55.66   10.273%   $61.378   $61.38   $61.40   $61.40  
  Purchase #7 $29.80   3.400%   $30.8132   $30.81   $30.80   $30.80  
  7 purchase totals = $761.68       $838.8909   $838.90   $838.90   $838.80