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 $5.37   3.242%   $5.5441   $5.54   $5.55   $5.50  
  Purchase #2 $174.59   8.645%   $189.6833   $189.68   $189.70   $189.70  
  Purchase #3 $162.62   8.803%   $176.9354   $176.94   $176.95   $176.90  
  Purchase #4 $32.42   12.289%   $36.4041   $36.40   $36.40   $36.40  
  Purchase #5 $29.80   3.110%   $30.7268   $30.73   $30.75   $30.70  
  Purchase #6 $49.35   4.380%   $51.5115   $51.51   $51.50   $51.50  
  Purchase #7 $143.66   5.854%   $152.0699   $152.07   $152.05   $152.10  
  7 purchase totals = $597.81       $642.8751   $642.87   $642.90   $642.80