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 $218.63   3.289%   $225.8207   $225.82   $225.80   $225.80  
  Purchase #2 $129.74   6.448%   $138.1056   $138.11   $138.10   $138.10  
  Purchase #3 $158.79   12.786%   $179.0929   $179.09   $179.10   $179.10  
  Purchase #4 $133.71   5.429%   $140.9691   $140.97   $140.95   $141.00  
  Purchase #5 $167.55   11.159%   $186.2469   $186.25   $186.25   $186.20  
  Purchase #6 $105.46   9.723%   $115.7139   $115.71   $115.70   $115.70  
  Purchase #7 $11.55   6.857%   $12.342   $12.34   $12.35   $12.30  
  7 purchase totals = $925.43       $998.2911   $998.29   $998.25   $998.20