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 $14.71   12.179%   $16.5015   $16.50   $16.50   $16.50  
  Purchase #2 $158.99   4.812%   $166.6406   $166.64   $166.65   $166.60  
  Purchase #3 $139.90   11.240%   $155.6248   $155.62   $155.60   $155.60  
  Purchase #4 $143.28   3.667%   $148.5341   $148.53   $148.55   $148.50  
  Purchase #5 $80.97   4.655%   $84.7392   $84.74   $84.75   $84.70  
  Purchase #6 $230.42   6.443%   $245.266   $245.27   $245.25   $245.30  
  Purchase #7 $167.93   6.211%   $178.3601   $178.36   $178.35   $178.40  
  7 purchase totals = $936.20       $995.6663   $995.66   $995.65   $995.60