pH will have the biggest swing according the the time of day and lighting. Everything else should be fairly stable. When I first started out I made it a point to test at various times to gain knowledge about the variations.
If you are looking for consistency test at the same time every day.
PH is usually lower in the morning because lights were off during the night. I do mine about 4-5pm. Different times will not affect your other test unless you are dosing more at one time of the day. I do like to be consistent though on the time of day.