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 $15.40   3.639%   $15.9604   $15.96   $15.95   $16.00  
  Purchase #2 $87.36   3.375%   $90.3084   $90.31   $90.30   $90.30  
  Purchase #3 $107.25   13.434%   $121.658   $121.66   $121.65   $121.70  
  Purchase #4 $51.70   10.153%   $56.9491   $56.95   $56.95   $56.90  
  Purchase #5 $172.27   11.104%   $191.3989   $191.40   $191.40   $191.40  
  Purchase #6 $154.83   12.430%   $174.0754   $174.08   $174.10   $174.10  
  Purchase #7 $157.85   10.189%   $173.9333   $173.93   $173.95   $173.90  
  7 purchase totals = $746.66       $824.2835   $824.29   $824.30   $824.30