It would depend on the selenium levels in the area you live and more importantly in the feed (if it's from out of your area) as the total amount of selenium needs to be taken into context - Pasture, hay, grain, supplements, etc.. Selenium toxicity is just as bad as a lack of selenium.
We keep selenium blocks out 24/7 for our horses along with mineral blocks and plain white salt (not that the selenium blocks get uses much!). We're on hills and our selenium levels are apparently good (according to my vet) but our hay is grown in areas with low selenium.
But we have administered in the past Selenium/Vit E to mares with the pre-foaling shots. I discuss it with my vet (and get it from him) before we do. Also Sel/Vit E comes in different strengths so make sure you know what strength you're administering too!
There are selenium maps on line if you want a generalization about your area.