Perfect form is the most important thing to have a perfect body, it's impossible to make a painting with a big brush
My training was based on concentration, and it's impossible to cheat when you are concentrated
Yes it is better to keep training even if your muscle is still sore.
To be tired it is only in your mind.
I do for one full hour non-stop of 2,000 sit ups and then 6 sets of leg rise.
If you don't like don't do it. But don't forget that: NO PAIN NO GAIN.