how do you deal with your variables?

[eluser]batteries[/eluser]
I don't mind passing variables around in a "procedural" fasion.. But, when dealing with a complex execution, i will use a more oop approach and set my data into the relevant model as private variables.

So my question, how do you guys do it? is there a best practice? For my current project i want to deal with my data exclusively in the oop way but i'm not sure if i should validate before i set the data to variables, or to set the data as soon as i get it..

Thanks