my point of view (as graphic designer in print templates) -> 300DPI for print designs is a must.
for example - design of A2 poster which is 426mm x 600mm (including 3mm bleeds top/bottom/left/right) is in 300DPI - which is 5031px x 7087px - this is normal 300DPI dimensions for print purposes.
If you make photo in the same dimensions which are A2 poster size 426mm x 600mm (including 3mm bleeds top/bottom/left/right) but in 72 DPI the final size in pixels is 1208px x 1701px - and now I can not use this picture in my printready A2 poster design because it is too small, have too low resolution.
So If you want sell your photos for print designs always check your photos in 300DPI resolution.
I can tell you basic pixel dimensions for main / popular flyer and posters sizes as an example:
A4 size for print in 300 DPI is about 2551px x 3579px
A3 size for print in 300 DPI is about 3579px x 5031px
A2 size for print in 300 DPI is about 5031px x 7087px
If we design something big - for example banner 5 meters width x 1 meter high we can use photos in 120 DPI no problem (biger size = lower DPI but starting from really big sizes, posters I mentioned above should be always 300DPI)
this is about photos for print designs - for my small poster A4 I need much bigger photo than web designer for his online banner - so my answer is about printing industry (which is quite big and shouldnt be ignored for sure by photographers
This is short answer if you have questions feel free to ask ;]