As I said earlier it depends on the LPI value of your otput device, so if you intend to utilize the output defice to the full then use 1.6-2,2 times the LIPI value. Anything larger than that just wastes space and prolongs printing...
Anything less on the other hand will lower your quality.
But 300 DPi is fine for most jobs but i have had to do images with 600 (a3 size) dpi quality, for a fotometric sublimation printer (this is kindof overkill because a sublimation printer wont rasterize colors at all), so the point is that you cannot know the resolution needed unless you know what it will be outputted for and what it will be used for. Period.
PS its a good idea to use sublimation printers for your portfolio pictures, yeah its a bit more expensive but the quality cant be beat.