If a baby is being supplemented because it has been ascertained that baby needs supplements, either because of low milk production or ineffective suckling, the idea is to supplement as needed but to maximize time at the breast as well. So you want to encourage comfort nursing-baby finding comfort as well as food at the breast. This can be done by allowing baby to 'hang out' at the breast, fall asleep at the breast, etc. These behaviors are normal parts of normal breastfeeding and are thus helpful behaviors to encourage in a baby that you eventually hope to fully or mostly breastfeed. This would mean, giving baby a small amount of supplement at a time, which amount would vary slightly according to age and how much baby needs to be supplemented, and then letting baby "finish at the breast."
When instead, baby is nursed and then supplemented, it may lead to a few problems. For one thing, nursing session may be cut short in order to make time for the bottle. Another is baby may be given a bottle or given more supplement than needed, because baby 'seems hungry' or 'seems unsatisfied" rather than put to the breast again. And baby learns to equate satiety with the bottle, rather than the breast.
What will work best will depend on the individual situation. It is fine to be flexible and see what works best in a particular situation. But the overall point is to give baby enough supplement rather than too much, so as to, in every way, encourage as much time at the breast as possible.
To often, when a baby is supposed to be given "supplements" - they actually end up getting all or almost all their nutrition at the bottle, which will lead to a needlessly premature end of breastfeeding. So how a supplement is given is very important.