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 $21.89   3.665%   $22.6923   $22.69   $22.70   $22.70  
  Purchase #2 $216.91   4.692%   $227.0874   $227.09   $227.10   $227.10  
  Purchase #3 $103.50   11.522%   $115.4253   $115.43   $115.45   $115.40  
  Purchase #4 $108.78   8.507%   $118.0339   $118.03   $118.05   $118.00  
  Purchase #5 $233.58   9.603%   $256.0107   $256.01   $256.00   $256.00  
  Purchase #6 $72.79   8.243%   $78.7901   $78.79   $78.80   $78.80  
  Purchase #7 $19.94   11.795%   $22.2919   $22.29   $22.30   $22.30  
  7 purchase totals = $777.39       $840.3316   $840.33   $840.40   $840.30