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 $37.30   5.301%   $39.2773   $39.28   $39.30   $39.30  
  Purchase #2 $135.82   12.437%   $152.7119   $152.71   $152.70   $152.70  
  Purchase #3 $164.54   3.614%   $170.4865   $170.49   $170.50   $170.50  
  Purchase #4 $103.39   10.503%   $114.2491   $114.25   $114.25   $114.20  
  Purchase #5 $108.93   11.587%   $121.5517   $121.55   $121.55   $121.60  
  Purchase #6 $132.82   3.725%   $137.7675   $137.77   $137.75   $137.80  
  Purchase #7 $159.55   10.701%   $176.6234   $176.62   $176.60   $176.60  
  7 purchase totals = $842.35       $912.6674   $912.67   $912.65   $912.70