[Vern]

In EP or MP1 I think I limp behind every time unless the table is very tight and I can get heads up with a weak opponent that limped in front of me. Since that is rarely the case, limping behind is likely the 99% answer. After MP2, you will open raise with just about anything worth playing because of the chance to take the blinds right there. EP and MP1 at a full table are different and table dependant with middle pairs. Will your raise get you heads up with a blind or is the table loose? If it is loose, limping in early may be ok since you don't mind as much if someone raises behind and isolates you to make it heads up or if 5 others limp. You just play post flop differently depending on what happens. The fact that it is folded to you in EP/MP1 has no real bearing on open raising. At MP2 it is a read call but generally should be played like MP3 and beyond where everything is raise or fold if you are opening.

The book itself discusses hands in MP that should be open raised because there are two ways to play them, HU unimproved or multiway (5+ way) improved to powerhouses but that don't thrive in the inbetween of 3-4 way action. Your middle pairs fall into this realm.

Vern

Standard Disclaimer, I am by no means an expert at any of this.