As pointed out above, tracking weight is very important as it might be one of the showing signs that your bird is sick before anything else starts showing up.
I typically weigh my guys every second day, although since Léa's arrival they've been weighed once a day, every day and Léa has been weighed three times a day, every day (as she is still growing and weight fluctuates).
It's also important to weigh them at a similar time - as weight with an empty/full crop varies, pre/after morning poop varies..you want to do it fairly constantly.
Close weight monitoring has shown me that when I go away and have my birds looked after can be "bad" for Piper as he generally has a drop in weight. The fact that this drop always happens for the same circumstance at least shows me it's likely not a disease as the weight goes back up once he's back home with us.