Depends on your dc.
I go if the illness is either out of their normal routine for illness, or I think they may need medication.
With dd2 her temperatures rarely needed anything other than over the counter stuff. but her temperature regularly hit 40, occasionally 41, and would stay there for 3 days before falling slowly. If it followed that path I didn't worry (well perhaps the 41s I got a bit concerned).
Ds's temperature I worry if it gets over 39. But for tonsillitis he goes into what I describe as a complete collapse. He tucks himself up into bed and sleeps with a mild temperature for 24-48 hours. He won't drink (much, I usually manage to syringe a few ml of water in), if he needs to get up to the toilet he crawls, and is very difficult to rouse at all.
Dd1 always downplays her illnesses and wants to go to school (not missed school now in over 7 years).
If dd1 said she didn't think she was well enough to go to school, I'd probably be rushing her to A&E... with the other two I'd tell them not to be so silly and the school will send them home if they're ill.
If ds had a temperature of 40, I'd get it investigated; if dd2 was unresponsive with a mild temperature, I'd be worried.
But this is my experience with each individual child has shown that this reaction is normal for them. and doesn't need treatment. It would not be right to ignore those symptoms for another child.