2 X the linescreen is the best rule, you can go to 1.8 X linescreen if you need to and get good results. It's sampling theory you're dealing with here. 300 DPI is OK and usually considered a standard but it's actually a bit high if you're running a 133 LPI screen which is very common.
Lots of newbies think the higher the resolution the better but that's not the case. An image which has too high a resolution actually results in a a muddier image, less contrasty. Sometimes that's what you want but usually not.
If you do a lot of printing you can verify this for yourself, try some low resolution images, maybe 120 dpi and you'll see how bright they look but at the expense of detail. Then try the opposite.
I used to own a print shop so I've had lots of practice and made plenty of mistakes.