IF you are using plugins that emulate vintage hardware and you are recording/mixing in 24bit, then the "sweet spot" is said to be -18dbfs and it's suggested to use the trim on your DAW (or a trim plugin first in your fx chain) since the signal is prefader going into the plugins. This is from Graham at the Recording Revolution:

http://therecordingrevolution.com/2013/11/25/do-you-know-how-to-read-your-meters/

And I've read long discussions about that from some other places too.

But personally, I just keep the peaks somewhere between -12 and -6. I use Realtracks and rarely record anything other than vocals. And i found the above advice (-18dbfs)to not work well for me mixing with the FX I use. YMMV

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