At what temperature would I be concerned? topic

Like others, my Nexus 9 gets fairly hot when playing games (Hearthstone), watching videos and browsing the Web.
I installed an app to monitor the temp, it usually spikes at ~70°C. I'm used to paying attention to temps on servers and Pc's. In that context I would be concerned about a machine that was at that high of a temperature, but I realize a tablet is a different beast.