Launch everywhere at the same time or day by day?

This of course depends on what you're launching but what Ive done historically was a reddit based strategy. I'd find 4-5 relevant subreddits and then write 2 versions of the post on the two smallest of the /r 's I found and see which one performed better. Then the next day launch it on the medium ones (again seeing which performs better) and then the third ay launch it on the biggest relevant subreddit with your best performing copy