No, I don't use the manual focus with the shutter pressed in halfway. I have found that if I use the manual focus to get the subject in perfect focus, and then push the shutter down halfway, the subject may become somewhat blurry depending on the settings I am using (expecially when using the macro lens). If this happens i do one of 2 things.
1) with the shutter still pressed in halfway, I move the camera slightly further (or closer) from it's originial location until the focus looks correct.
2) I use manually focus to set the subject slightly out of focus, and then when I press the shutter down halfway the subject becomes more in focus.
I find method #1 much eaiser.....this really only happens to me when using the macro lens, and with the macro lens I don't rely as much on manual focus as it is much more effective to just move the camera further from or closer to the subject with the shutter held down halfway to get the proper focus.
Also, I prefer to not use flash, so I generally can't get away with ISO 80....depending on how well lit the subject is I'm generally at 100, sometimes even 200. (Wish there was a 150)