How big of a deal are off-target effects in CRISPR editing in the lab?

[StevenTJ]

I'm an undergrad in molecular biology, and we're starting a module on CRISPR gene editing next week. The potential is obviously huge, but I keep reading about off-target effects and ethical debates that make me a bit uneasy. For researchers actually using this technology in the lab, how much of a practical hurdle are those off-target edits right now? Is it something you're constantly checking for, or have the techniques gotten precise enough that it's less of a day-to-day worry?