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 $19.75   4.431%   $20.6251   $20.63   $20.65   $20.60  
  Purchase #2 $166.41   6.114%   $176.5843   $176.58   $176.60   $176.60  
  Purchase #3 $34.34   6.818%   $36.6813   $36.68   $36.70   $36.70  
  Purchase #4 $36.90   4.153%   $38.4325   $38.43   $38.45   $38.40  
  Purchase #5 $107.67   3.387%   $111.3168   $111.32   $111.30   $111.30  
  Purchase #6 $9.57   3.736%   $9.9275   $9.93   $9.95   $9.90  
  Purchase #7 $225.95   9.617%   $247.6796   $247.68   $247.70   $247.70  
  7 purchase totals = $600.59       $641.2471   $641.25   $641.35   $641.20