It's not any different in Reaper than it is in any other DAW. It all depends on what you wanna do with the stereo mix. If you plan to send it off to mastering, then you need to find out how your mastering guy wants it.
Don't think so. If it was to go through another step -mastering, then use the lower levels... Am I missing something?
When you say -6 and -4, do you mean -0.6 and -0.4?
Would it be safe to say that for 'to be mastered mixes' part of the reasoning for a general target range of -10 peak or a bit higher be to accommodate future possible analog processing?I'm sure he meant -6 and -4 since your post suggests that you're talking about mixing. I even find that a little high, but we're splitting hairs. I get my mixes to peak at about -10. This is for the MIX, not the final master.
If you're doing your own "mastering", as many of us home-recorders do, then you're probably talking about getting it to peak at about -.2, while the average level is around the -12 to -13 mark. This is just my personal way of going about it, so I'm not saying all final "mastered" mixes have to be at those levels. But that's what usually works for me.