It’s a good idea to ‘sit one out’ occasionally in class. The most important aspect of our hatha yoga practice is being as mindful as possible, paying close attention to what you’re doing and how it feels. Working in a hot room can be challenging and if you practice a posture while you’re fatigued it will be more difficult to be fully present and alert to the subtleties of alignment and depth.
If you continually practice postures while fatigued you are more likely to reinforce bad habits and less likely to be safe. Most injuries occur through neglect (not paying attention) or over-exertion (pushing yourself farther than your body is ready to go). You will get a lot more from your practice if you regulate your exertion level and don’t allow yourself to become fatigued.