600dpi is a bit overkill -- unless you are printing high quality photographic prints and using the best paper for the job.
Keep in mind that if what you're printing hasn't been "brought in" at 600 dpi, even when you set your printer to print 600dpi, you won't get the quality you're looking for. For example, say you uploaded a picture from your camera. Depending on how your settings are on the camera, it may show up in the program you're using as a 13x20 picture @ 150dpi.
You then do nothing but tell the program to print the picture to fit 5x7... it will still be at a lower resolution unless you change the dpi to 600 AND change the settings on the printer to print at 600dpi.
Anyway, 300dpi is usually fine for most photographic-quality printing - on 'better' paper. 150dpi, believe it or not, is usually fine for text. Once again, paper is a big factor. The better the paper, the better the print quality. Most people will tweak their settings and use the cheapest paper out there.. and end up with that 'rippled' paper effect because the higher the printer settings, the more ink it uses. Cheap paper doesn't have the layers of coating that the better paper does. The ink is designed to 'seep' into some of the layers and stay on top of all but the outmost layer of good paper.
I've always recommended buying 3 reams of paper: one really cheap (like $3) for draft mode printing and the like; one "inkjet quality" ream (usually about $7-9)- which you would tweak your printer up to no more than 300dpi; and one "really good" photo type ream (or box) for the 'good stuff' (300-600dpi). In the end, you'll save a ton of money on ink and paper since you're only using what you need for each specific print job.
I know, TOO much info.. but it's really how it should be!
Hope this helped!
Maggie B