Check it every time. I've had units that were fine for months or years, and then one day were off by 20 or 30%. No mishandling, no abuse, no nothing wrong.
Make sure you understand the procedure for use, as well. The common ATC (auto temp correction) units need the sample to sit on the glass for 45 seconds before you take a reading. If your isn't ATC, you need to understand the temp and/or procedure it's designed for.