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 $15.80   13.531%   $17.9379   $17.94   $17.95   $17.90  
  Purchase #2 $8.80   9.371%   $9.6246   $9.62   $9.60   $9.60  
  Purchase #3 $52.41   9.690%   $57.4885   $57.49   $57.50   $57.50  
  Purchase #4 $148.73   8.509%   $161.3854   $161.39   $161.40   $161.40  
  Purchase #5 $9.39   8.673%   $10.2044   $10.20   $10.20   $10.20  
  Purchase #6 $214.41   7.552%   $230.6022   $230.60   $230.60   $230.60  
  Purchase #7 $41.55   6.609%   $44.296   $44.30   $44.30   $44.30  
  7 purchase totals = $491.09       $531.5390   $531.54   $531.55   $531.50