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 $249.90   5.226%   $262.9598   $262.96   $262.95   $263.00  
  Purchase #2 $116.99   9.691%   $128.3275   $128.33   $128.35   $128.30  
  Purchase #3 $38.85   11.130%   $43.174   $43.17   $43.15   $43.20  
  Purchase #4 $28.41   7.528%   $30.5487   $30.55   $30.55   $30.50  
  Purchase #5 $158.99   3.183%   $164.0507   $164.05   $164.05   $164.10  
  Purchase #6 $5.81   9.675%   $6.3721   $6.37   $6.35   $6.40  
  Purchase #7 $140.58   5.271%   $147.99   $147.99   $148.00   $148.00  
  7 purchase totals = $739.53       $783.4228   $783.42   $783.40   $783.50