Where the needle enters is half the decision; how it travels is the other half. Most piercings are placed perpendicular to the tissue so jewelry sits level and pressure spreads evenly — an angled channel makes a stud tilt, a hoop hang crooked, and healing tissue carry uneven load, which is a classic irritation-bump recipe. Curved placements like rooks and daiths instead follow the fold, which is why they use curved barbells and rings.
Gauge — the thickness of the jewelry, where a smaller number means thicker — is decided at placement time too, and it isn't cosmetic. Standard choices exist for a reason: 18g–20g for nostrils and lobes, 16g for most ear cartilage and lip placements, 14g for navels, tongues, and industrials. Thicker jewelry in high-stress spots resists migration and tearing; too thin acts like a cheese wire under pressure.
Depth matters as well: too shallow and the body can push jewelry out over time (the failure mode surface piercings and eyebrows manage carefully), too deep and it sits buried and swollen. You can't see any of this in the mirror on day one — which is precisely why piercer experience is what you're paying for.