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 $38.27   4.430%   $39.9654   $39.97   $39.95   $40.00  
  Purchase #2 $45.73   11.281%   $50.8888   $50.89   $50.90   $50.90  
  Purchase #3 $114.94   11.749%   $128.4443   $128.44   $128.45   $128.40  
  Purchase #4 $71.50   8.524%   $77.5947   $77.59   $77.60   $77.60  
  Purchase #5 $64.96   8.186%   $70.2776   $70.28   $70.30   $70.30  
  Purchase #6 $121.73   6.738%   $129.9322   $129.93   $129.95   $129.90  
  Purchase #7 $80.36   11.862%   $89.8923   $89.89   $89.90   $89.90  
  7 purchase totals = $537.49       $586.9953   $586.99   $587.05   $587.00