As a rule, should playing open strings be avoided?

it depends. do you like playing vibrato (then don't play open strings) otherwise go ahead and play them

very good answer - thank you

I seem to find that open strings should be avoided if holding a note for an extended amount of time, such as semibreves, as it starts to sound 'tinny'.

Yes, I agree.

if it's D E D E or something like that (on the A string), play with the fourth finger. At least that's what I do.

It depends on the song youre playing too. But with most upper level songs you should never use an open string unless its a fast run/scale/phrase. But you should avoid it whenever possible

For the most part, avoid open strings. Although as mentioned, in faster runs they are ok. But especially avoid open E as it sounds so awful!

Yes, I agree. The violin was never intended to play open strings.

I agree, it should be avoided, but if you can't get your 4th finger in tune it sounds better with an open string.