Yes, playing open strings is OK. And yes, I do it too. But (and the caveat is important) there may be times when it is inconvenient to play the open string, for instance when the melody goes back down the scale and it would be, therefore, easier to eliminate an unnecessary string crossing. Also, you can get some nice tone coloring if you play a fingered note with the fourth finger, with the adjacent open string, example, finger an A on the D string, and play the open A string at the same time. At phrase endings where it is practical, this is very effective.