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:
Again- not harmful, but not necessarily helpful, either.
Well it’s not just the dpi that matters! You could have a 200 dpi image that’s only 400 x 400 pixels, and you’re still going to get some pretty shitty artifacting from your strokes. :)
I work at 8.5 x 11”, 300 dpi at the least, but most of my finished pieces are 11 x 17” and up. This is some of my lineart at 100%, par exemple:
high resolution is a must, even if you’re just downsizing it and posting it online- it allows you a sharpness and level of control that you can’t get at basic screen resolution. You can get away with 150-200 dpi, I’m sure, but I wholly suggest upgrading to 300 and up!
It’s also awesome if you ever need to resize your work for promotional purposes- if you’re working in the 300+ dpi range, you have a lot more leeway for what’s going to look good on a giant banner. :D