I too use placidus, probably because I first tried Koch when I wanted to "chart" my own chart. I found that it didn't feel right to me. Astro.com uses placidus, obviously the older and generally more revered system, and that is how I came to use it. It made more sense to me and I went with it. I do agree with leftie....you have to use intuition or you won't get all that far in this field. Indeed, the Koch system won't make you better or worse. I just always prefered Placidus, but that's just me.
I know that many astrologers recommend Koch for a beginner and then move on the Placidus, just because Koch is easier for most to read and understand. When you're starting out and someone says "hey, by the way, not every house is 30°....actually in some birth charts none are! Good luck now."....you'd probably be confused. I just think that although using Koch isn't bad or less correct by any means, I feel that Placidus gets the core of astrology better. If you live on the equator, there is no difference between Koch and Placidus (well, basically...). However, I don't recall having someone give me there info and them saying "I was born on the equator". Certainly it's possible but not likely. When you start moving, that's when you see the houses change and degrees change as well. So all in all, neither is wrong, but I prefer Placidus.
Interesting Q Isabelle.... You never fail to keep my mind goin!
edit: Yes, my venus is in either th 4th or 5th, depending on if I use Koch or Placidus. I personally feel more like a venus in the 4th person, and it can be tricky with aspects. Also, my MC changes to libra with the Koch system, and libra MC is not me. There is a book, I believe it's called "The Astrology Sourcebook"...anyway it is very good, you might want to check it out. Also, you can post a separate question asking about books on the topic....see what you get. I'll edit more if I think of another book, or if I find one of my own that describes it.