I felt the need for a solar plexus tattoo too, but I do hope to be skipping about naked at 80, well, if I fancy it. The standard underboob placement was never going to be optimal on me, I've never had the right sort of breasts. So, my tattoo comes up and onto to solar plexus, rather than being placed around it, if you see what I mean. It ends just, just under the band of my bra/bikini.
But, I'd say, with breasts like you describe @TrixieFaux, it's not boob saggage that's a potential issue with this placement, that's never going to mess things up too much. Rather, it's the way the underboob placement tends increases the visibility of the natural asymmetry of the breasts (that all women have). That asymmetry does become more apparent as the breast softens with age and reproduction even if there isn't significant sagging. Even on the very young, pert, instagram chests I've more than often seen the placement emphasise the asymmetry, making the tattoo look slightly wonky even though, technically speaking it is perfectly centered etc. which is a shame.
With mine being designed and placed as it is my breasts (when roaming free) actually look more symetrical than they did before, rather than less. Which is nice. I've posted it on latest tattoo lowdown already but here it is again.
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With asymetry, I mean the often subtle differences in roots of the breasts, rather than the slight differences in size and shape between the breasts (which we are usually aware of). The differences in height of the base of the breast and the precise distance from the from the center of the breast bone to the root of the breast can be pretty much imperceptible on a lot of women, until you place something that is utterly symmetrical in between and under them...