Yeah, 300-600 DPI is perfect for printing your comic however online posting a 600 dpi image isn't ideal since it's a larger image file and if the image is stolen you're given the thief printable files. Smaller images = faster loading time = good
Also there's no visual difference between posting a 72 dpi image and a 600 dpi image.
Web browsers show the ACTUAL pixels and the DPI is nothing more than an instruction that is used when an image is printed. A web browser doesn't care and just displays the image at screen resolution - usually 72-96dpi.
So it's good to create in high resolution and resize for the web when you're ready to post online