For DSLR, no harm keeping the camera switched on the whole day as long as the Live View is not activated.
Anyway, for DSLR as most have already said, boot up relatively fast, hence many switch off the DSLR when not in use and switch it on when shooting.
For Mirrorless, definitely no no. The system will heat up, and you sure can feel it. Battery drainage is high.
Cheers
Steven Yee