I might not be the best person to ask. The only time I feel it’s necessary to use a really high DPI on an image when engraving is when I’m engraving “Glowforge” on a grain of rice. (No kidding.) 
High DPI is important for getting realistic resolution in printed photographs, but I believe it’s actually detrimental in engraving with a laser, because it adds a lot of unnecessary data (causing hang-ups in the buffer and unspecified error messages). I discuss it a little bit in the tutorial linked below:
I generally try to keep the DPI at 300 or less, but if you have dragged a raster (bitmap) image into a program like Illustrator or Inkscape and it started out at 600 DPI, it will be loaded into the SVG file at 600 DPI. (The vector programs can handle them, it just takes a lot longer to load up.)
If you want to increase the “fine” look of the engrave, what you will want to change is the LPI. (Lines per inch.) But be careful not to go too high with those either - increasing the LPI above 195 to 225 causes the machine to burn over areas it has already burned over, so it deepens the engrave and causes more charring, instead of making the image look better.
YMMV. 