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 $17.93   3.429%   $18.5448   $18.54   $18.55   $18.50  
  Purchase #2 $249.50   3.114%   $257.2694   $257.27   $257.25   $257.30  
  Purchase #3 $7.80   12.392%   $8.7666   $8.77   $8.75   $8.80  
  Purchase #4 $216.51   10.128%   $238.4381   $238.44   $238.45   $238.40  
  Purchase #5 $220.63   6.300%   $234.5297   $234.53   $234.55   $234.50  
  Purchase #6 $171.51   13.864%   $195.2881   $195.29   $195.30   $195.30  
  Purchase #7 $156.25   4.195%   $162.8047   $162.80   $162.80   $162.80  
  7 purchase totals = $1,040.13       $1,115.6414   $1,115.64   $1,115.65   $1,115.60