I'd definitely test before eating or drinking anything other than water first thing, then again before snack and probably before breakfast unless they are both fairly close together. But you also do need to think about why /what you're testing for at any one time.
I can get a liver dump just by getting up, going to toilet, then going down to kitchen to put the kettle on, so do any fasting tests before I actually get up. And I must admit I can have a snack and not test, but may then see a higher than expected reading when testing before my next meal.
Robbity