The rule that decides everything: readability over time
Every question about lettering tattoos comes back to one fact: ink spreads slightly under the skin as years pass. In a rose that's invisible. In a word, it's the difference between a quote and a smudge. Small letters with tight loops, the e, a and o of cramped cursive, close up first, and a phrase that looked delicate fresh can read like a barcode in five years.
The fixes are size and spacing. Keep letters at roughly 1 to 1.5 centimetres minimum height, give them room to breathe, and be suspicious of any design you can't read from across the room while it's still on paper. A good lettering artist will push back on tiny text; that push-back is the expertise you're paying for. Cursive versus block matters less than people think: cramped cursive is the worst-aging option, but well-spaced script and clean block letters both go the distance.
Where words last, and where they don't
The forearm is the default for a reason: flat, stable, low-friction skin that you can actually read yourself. Ribs, the collarbone and the spine suit longer quotes and are popular; know that all three hurt more and cost more, since curved, bony areas are slower to work on.
The placements to avoid are the same offenders as every fine style, but worse, because a half-faded word is more obviously broken than a half-faded flower: fingers, the sides of fingers, hands, feet and the inner wrist. Knuckle words look sharp for months, not years. If you want text somewhere high-wear, go bolder and bigger than you originally planned, or make peace with re-touching it.