do what is best for you... if you wantn to enter any contests, make them individual diaries... you can always take on old multi-strain diary, download images and recreate individual diaries too.
so, it just depends on what you value and what you want out of it... if it's just to keep track for personal reasons, then keep it simple and reduce needless work - like 10 diaries for 1 grow isn't necessary if you just want a picture log history and some weekly notes.
When in get promotional seeds, i do those as individual diaries. I am not obligated to do so, but if they send the seeds, i'll put up some advertising for them in a more clear form... as a spectator, you definitely get more useful info and good looks when a strain is the sole focus of a diary.
but, i also only do 1 writeup for it all and copy/paste into each.. so it is only a matter of oranizing pictures and a few extra clicks to create weeks for multiple strains. I don't worry too much either way.. the most recent diary is a multi-strain diary. Some do a multistrain as an 'index' and individual diaries too.
Make it work for you... every context can be different.