Should you practice the notes which you can reach but feel strain or only practice with those where you are comfortable?
I apologize for any mistakes in the terminology. I am a beginner in music theory.
You mentioned the term strain. Strain is a big problem in the voice; if you feel strain, you should stop immediately. Pain is a message from your body telling you to stop.
You also mentioned that you don't know precise terminology, however. You said strain, but is it possible that the note is simply high and it is difficult for you to sing it? Depending on what you meant by strain, either you should be working on singing those notes to improve your range/voice, or you absolutely should not continue.
Two big things to take away from this thread: Stop if it hurts, and see a vocal instructor of some sort. Strangers on the internet (as qualified as we often are) won't be able to help you very much with this situation; ideally, in-person meetings with someone who can teach you singing technique would be preferable.
I would say no. If you cannot hit it without strain in is not really in your range. If you think the note should be in your range and for some reason it is straining on a particular day you might not be supporting the note properly, or have some issues with a cold coming on etc. Try lip trilling the note or some other resonance or vocalization exercise before "singing" it.
Avoid strain. Practice recognizing the difference between strain and discomfort. A good stretch can feel a little bit uncomfortable and moves one closer toward reaching what may be currently out of reach. Stretching in and of itself is a good practice to incorporate into your musical occupation.
In year forty-something of playing music, there still seems to be an endless amount of relaxation toward which to reach.