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 $33.76   13.171%   $38.2065   $38.21   $38.20   $38.20  
  Purchase #2 $182.96   10.637%   $202.4215   $202.42   $202.40   $202.40  
  Purchase #3 $16.73   11.267%   $18.615   $18.62   $18.60   $18.60  
  Purchase #4 $80.75   4.205%   $84.1455   $84.15   $84.15   $84.10  
  Purchase #5 $64.99   3.329%   $67.1535   $67.15   $67.15   $67.20  
  Purchase #6 $217.98   12.526%   $245.2842   $245.28   $245.30   $245.30  
  Purchase #7 $127.31   5.582%   $134.4164   $134.42   $134.40   $134.40  
  7 purchase totals = $724.48       $790.2426   $790.25   $790.20   $790.20