I can’t imagine any instance where that would be “harmful,” per sé, but it also seems like you’re dealing with diminishing returns at a certain point.
The only times I find myself using 900+ dpi is when I’m scanning something that I plan on enlarging for print or display- if you have long term plans to zoom into/enlarge your piece then knock yourself out, but 900 dpi seems a bit egregious for day-to-day work. My big takeaways:
- 11 x 17” at 900dpi is the equivalent of a print-resolution 33 x 51” file, and that’s gonna most workstations slow to a crawl if you’re working with any substantial number of layers
- brushes and brush textures have a set resolution (they aren’t vector), and they can start to look crappy when they’re enlarged past a certain size
- there’s no significant difference between printed 300 and 900 dpi to the naked eye, and the same can be said when you’re shrinking them down to 1000px tall for display online
Again- not harmful, but not necessarily helpful, either.