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 $13.39   13.818%   $15.2402   $15.24   $15.25   $15.20  
  Purchase #2 $8.83   5.576%   $9.3224   $9.32   $9.30   $9.30  
  Purchase #3 $98.70   6.421%   $105.0375   $105.04   $105.05   $105.00  
  Purchase #4 $179.67   11.341%   $200.0464   $200.05   $200.05   $200.00  
  Purchase #5 $117.48   9.757%   $128.9425   $128.94   $128.95   $128.90  
  Purchase #6 $36.33   4.673%   $38.0277   $38.03   $38.05   $38.00  
  Purchase #7 $8.50   6.741%   $9.073   $9.07   $9.05   $9.10  
  7 purchase totals = $462.90       $505.6897   $505.69   $505.70   $505.50