Stressed yeast aren't the best thing to work with, but it won't ruin your beer. Just don't enter it in any competitions :)
If your lhbs carries it pick up a copy of Jamil/ Chris white's book on yeast. Not nearly as boring as it sounds!
Are you using ALL extract? Adding any specialty grains? I know when I started with extract I had some of the same problems of what I called the "Twang". Couldn't explain it other than by saying it tasted like homebrew. haha. It went away after I started doing partial mashes or by steeping...
lots. lol.
ok on a serious note, just be methodical. if you can, write down all of the steps ahead of time (checklist) so you don't forget anything. that helps me out alot.