OK, so I decided to go ahead and do some math to figure out what the best way was to maximize rested xp when leveling alts. My original assumption, which I posted earlier, was that doing about 6 levels per character should about do it, but when I run the math, it comes out significantly less than that. One quick caveat - there are a couple of assumptions in here that are specific to me - one being that I play around 40 hours per week, and I'm also looking to level 8 more toons to 80, rather than a full 10. If your play time is higher or lower, or if you're leveling a different number of toons to 80, you should be able to adjust this math accordingly.
First, I'm starting with the fact, per Blizz, that rested xp accumulates at 5% of a level per 8 hours and is capped at 150% of a level, which takes 10 days.
Then assuming an 8 day played time to 80, it should take about 2.5 hours per level, so if I'm playing 6 hours per day on average, that also works out to 2.5 levels per day. So, between working on each toon, I want to advance a total of 25 levels. If I'm leveling 8 toons, then for the 7 characters that I work on in between, I need to do 25/7 or about 3 1/2 levels per toon. So let's call it 4. Note, that I'm using even numbers because that let's you go get training and park your toon in a major city for the rested xp.
Now, if you're starting from scratch, and the early levels tend to fly by, you may want to adjust this a bit. The total 10 days, works out to about 60 hours of playing time, divided between 7 toons is 8 1/2 hours per toon. Interestingly enough, that should work out to taking each toon to approximatley level 18 at first, and then starting the pattern from there. It may even make sense to increase it to 6 for the first pass after level 18 since the levels still go pretty quickly at that low level.
If you're starting from scratch and doing 10 toons, I don't think the math really changes all that much because of the Death Knight factor, and because I've rounded pretty significantly here.
So, to sum it all up, you'd take you're first toon to 18, park him, then move onto the next one until you get them all to 18, then go back to the first one and level him to 24, park him, and move throught the others, then go back to the first toon, level to 28 and then continue in 4 leve increments. At some point, it will probably make sense to switch to every two levels - either in Outland or Northend. Just keep tracking of your leveling time. If it starts getting closer to 5 hours per level, then switch to every two levels instead of every four.