Tis a strange one as some trusted and very good comp shooters i know swear by sizing, i have tried it a few times over the years with proven batches and found (for my barrels) that it didn't make any difference. For me pellet weighing makes for tighter long range groups,.....even more so recently as i'm finding that my preferred pellet supplier is starting to have wide weight variations compared to 5 years ago.
As with anything you would need a good quality set of scales to weigh and if sizing a good quality sizer.
Sometimes it may just be the placebo effect that when i get to a comp i know that my pellets are as good as they will be and we all know the positivity of sporting psychology.