Evaluating a script is, perhaps, the most important aspect of compositing.
Editing the color values of the composite requires that the compositor evolves a method of evaluating this colors. This feedback can be in the form of generalized visual feedback or something more specific like a set of number values. Nuke has a lot of tools that have either been designed specifically for this purpose or can be adapted to be used as such.
Working on an image it is easy for the eyes to get 'tired' of the script and to become blind to its pictorial shortcomings. The content of an image is not subject to numerical evaluation in the same way that color is. Here are a few ways to refresh those eyes:
Nuke is a very 'adult' application that lets you handle your material in any way that you wish. Unlike Photoshop, Nuke will not give you a warning beep or forbid you from doing anything. For this reason it is very easy to accidentally do things in the 'wrong' way. This is a short (non comprehensive) list of common 'best practice' items. Of course, like all 'rules', they are there to be broken.