It is preferable to edit natively. ie edit what you shoot in a matching preset.
Tell us what you shoot and also tell us what you generally export to (ie for what purpose, TV, Youtube, DVD, etc...)
Someone will advise a decent workflow. (Why am I sure you are going to say AVCHD)
BTW - it is often said that SD DV was the easiest workflow for Premiere ( and that is correct). Shoot miniDV and import it into a DV Preset.
Problem is . HD made this all a different ball game.
Converting Source material so one can edit it would be a real painful workflow unless you really have to because of your system and hardware limitations.
Along the lines of Craig's reply, if you tell us all about what you are doing and with what you are doing that, we can likely help you much better.
This ARTICLE will give you tips on the types of info that will be very useful. Personally, I find that if one talks in concrete terms, rather than the theoretical, useful answers are more forthcoming and more useful too.
I work with a TON of different formats, which is why I need to just one good codec to convert everything to. I don't export to just one format, at the moment. I usually make web video, which is most often H.264 avi's, or whatever works for downloads.
Does Premiere use different codecs for different project presets? (for preview renders, etc)
OK, are you starting with HD material, or SD material? The ultimate CODEC (if there is such a thing) will depend on that. Even then, one might be doing some conversions, outside of PrPro.
So, you can't just convert to the exact format that Premiere is using for the preview files? (I can open them in media players, so surely it's a common enough codec, I would think... something that at least GOM Player understands, or that comes with a major codec pack)
Premiere uses a preview CODEC dependent on the Project Preset. eg avi, dvcpro, I-frame mpeg, DV, ....
(Check them out by starting a new Projects and taking a look at the options)
In your case , using many HD formats in the timeline, you are stuck with rendering unless you want a massive exercise in converting every single asset and it still wont mean no rendering for playback previews.
"Play back" is a different kettle of fish to "editable"