You will absolutely be ranked higher on their list as well as more likely to be interviewed if you do a sub-I at their institution. On the other hand, if you aren't real impressive to them while you are there you can of course hurt yourself.
Most places will grant you an "automatic" courtesy interview if you do a rotation at their place (although this may not always be true in the more competitive specialties).
You do not need to schedule more than 2 away electives in any specialty. I only did 1 away and 1 at my home institution. BTW, a month away is a long time so be nice to yourself and minimize.