Not really. A well written compiler can optimize code better than what I could do if I wrote the bloody piece all the way by hand.
The only place where it really makes sense to do some manual optimizing is checking how the machine code looks when profiling a hotspot. And even then it usually turns out the compiler did the best job it can in that situation and the correct way to optimize is to refactor the c source

It might well be that I'm just so bad at this and your mileage might vary but this is what I have learned working almost 30 years writing device drivers...