I edit direclty from AVCHD all the time and it works fine for me on several systems including an older macbook pro. I suggest avoiding transcoding if possible. It's time consuming and if you don't do it right, you could overcompress and suffer generational loss.
I do like to go in and delete the unused clips after I'm done editing, just to save space. Also, if it's projects I don't care about, I will sometimes export the final movie into something easily playable outside of ppro (like mov or mp4) and then delete all the source media. That is NOT a best practice if you're working professionally. In that case, you need to archive everything. But for prosumers, there may be a lot of projects where you just want to put a quick edit together and then export it as a final movie for posting online or wherever. In that case, you may not want to hang onto all your source footage forever.
It really varies per user, but like I said, if you are working professionally, or aspiring to, you will need a way to archive everything, and to backup those archives. If you're working from tape source, then just labeling and storing the tapes is an acceptable backup for the media associated with a certain prject (of course you'll still need to save and backup the project itself). But if you're working from tapeless formats, then you will need multiple hard drives with lots of space to archive and backup everything. The good news is that hard drives keep getting cheaper, bigger and sturdier all the time. I've had two lacie 1 tb hd's for a couple years now with zero problems (before that it was 2 lacie 500gb's for many years, also with 0 problems). But I'm still always parnaoid that #1 will go while #2 is still in the act of backing it up. Or that they'll both go at once. Yikes.