It depends on three things:

1) What kind of work you tend to do and

2) How complicated the formula is and

3) How you are storing the formula.

1) If you are ceaslessly doing the same calculation over and over again, of course it makes sense. Everybody I know does this. However, if you are in school and you simply want to store a way to solve quadratic equations for you then I would recommend that you don't. The purpose of schoolwork is practice. Writing a program to solve the problems for you is good a exercise, but defeats the purpose of the homework.

2) If you have a simple formula for what you need then you can simply store it as a formula. For example if you are finding many points on the function sqrt{x^3 + 3x^2} + exp{sin(5x + 4)/x!} then, yeah, I'd write out the function on the screen and automate it. You should be able to copy and paste the line above and put a "|x = 3" (for example) after each line to evaluate it at a specific x value.

3) For a genuinely complicated formula you can go into the editor mode and define either a function or write a program to do it for you. For example if you are trying to find the exact solutions of a quartic polynomial, a simple process like I described in 2) is going to be impossible. But you can write a program to do it if you have the formula handy.

Unfortunately I have a TI-83 and a TI-92, not a TI-89. The 89 is similar to the 92 but I don't know how close the details are, so I can't help you with the specifics.

-Dan