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 $11.73   11.387%   $13.0657   $13.07   $13.05   $13.10  
  Purchase #2 $142.90   3.475%   $147.8658   $147.87   $147.85   $147.90  
  Purchase #3 $237.74   4.663%   $248.8258   $248.83   $248.85   $248.80  
  Purchase #4 $77.96   7.480%   $83.7914   $83.79   $83.80   $83.80  
  Purchase #5 $36.90   8.444%   $40.0158   $40.02   $40.00   $40.00  
  Purchase #6 $93.66   9.354%   $102.421   $102.42   $102.40   $102.40  
  Purchase #7 $201.93   5.672%   $213.3835   $213.38   $213.40   $213.40  
  7 purchase totals = $802.82       $849.3690   $849.38   $849.35   $849.40