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 $28.31   7.866%   $30.5369   $30.54   $30.55   $30.50  
  Purchase #2 $27.61   12.604%   $31.09   $31.09   $31.10   $31.10  
  Purchase #3 $26.66   12.566%   $30.0101   $30.01   $30.00   $30.00  
  Purchase #4 $161.59   5.279%   $170.1203   $170.12   $170.10   $170.10  
  Purchase #5 $14.98   4.113%   $15.5961   $15.60   $15.60   $15.60  
  Purchase #6 $28.68   11.812%   $32.0677   $32.07   $32.05   $32.10  
  Purchase #7 $199.76   7.372%   $214.4863   $214.49   $214.50   $214.50  
  7 purchase totals = $487.59       $523.9074   $523.92   $523.90   $523.90