Usually do 10 reps with half the weight I will be using, then 70% for 8, 90% for 3, then do my working sets. Never find my self becoming tired for my working sets either, and im prepared for them.
Stretching after each set is also a good way to help recovery and it makes your muscles stronger
Very similar to me. My warm up is 50%, 75%, 90% (6, 4, 2 reps) then onto working sets.
I can't remember exactly why, it's not bad for you as such, but releases energy or tension in the muscles causing you to lift less and be slightly weaker. Someone with the right explanation/knowledge can probaly explain this better than I can.
Stretching post exercise however is good
Yup you shouldn't stretch before a work out in the discipline of weight lifting. The research shows that you'll be weaker and from what I've read this is basically that stretching a muscle releases tension to help the muscle and connective tissue (fascia etc) relax. You don't want this before lifting weights, furthermore you'll have a greater chance of injury and also you'll be weaker in your lifts. This is for static stretching.
Certain types of stretching can be used for different sports but for weight lifting a different protocol is to be used.
Other research shows that it's actually best to stretch a few hours after weight training. There's a sequence to the types of stretching to be done as well, along the lines of: PNF, ballistic & then static. However I'm not an expert - I have a swiss ball at home and use it and just my bedroom furniture in general to help me stretch after a big session.
I have no flexibility issues for someone of my size, sure I'm not super bendy or can't do the splits, but I have no tightness in areas that shouldn't have them (i.e. hamstrings, shoulders,back ...( I do have a little on the neck, but that's an injury so I do what I can with it!
However a warm up is very important! It does make a difference to your performance, certainly for lower body exercises, where the mobility movements can open up your hips and loosen any tightness and improve ROM. Also it'll bring blood to your joints, and tendons and have them prepared for the warm up sets. I tend to do some start jumps, prisoner squats, rotating hydrants and shoulder rotations. If I have space I'll do some mountain climbers. It takes 5 mins and is very effective.