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 $53.43   4.826%   $56.0085   $56.01   $56.00   $56.00  
  Purchase #2 $135.76   12.499%   $152.7286   $152.73   $152.75   $152.70  
  Purchase #3 $231.68   11.382%   $258.0498   $258.05   $258.05   $258.00  
  Purchase #4 $237.58   4.857%   $249.1193   $249.12   $249.10   $249.10  
  Purchase #5 $143.95   6.238%   $152.9296   $152.93   $152.95   $152.90  
  Purchase #6 $165.37   6.750%   $176.5325   $176.53   $176.55   $176.50  
  Purchase #7 $250.92   10.808%   $278.0394   $278.04   $278.05   $278.00  
  7 purchase totals = $1,218.69       $1,323.4077   $1,323.41   $1,323.45   $1,323.20