"Position baking rack in the middle of the oven"
"Position baking rack in the middle of the oven" - does this mean that the rack itself is in the middle or that, when you place a cake pan on it, the cake will be vertically centered in the oven, thus the rack is positioned one slot lower? I never thought about this before. My oven has five positions for racks, so if I place the rack in the center position, the cake sits higher than center in the oven. I started to wonder if this will cause the cake to become more dried out on top. What do you do, and why, and what results do you get?
"Middle" of the oven, IMO, means the the item being baked needs to be within the center 1/3 portion of the oven. An inch or two one way or another from the "center" isn't critical. The reason for "centering" the item in the oven is to provide the highest possible balance in temperature. Convection ovens don't' suffer from the varied temperature zones in a typical oven so it isn't even a consideration for those types of appliances.
Don't be concerned beyond trying to maintain the center 1/3 rule.
Can you imagine, if were specified in a recipe, trying to hold a 20 pound turkey in the "center" of your oven. That ain't gonna work. So the best you can do, if it were even an issue, would be to align the center line of the turkey at or near the center line of the oven. Therefore, even for the turkey, the center 1/3 satisfies the rule.
I've never been super-exacting about this either way (in fact, I very often forget to check what positions my racks are in until the oven's already hot, and then just sort of go with what I've got rather than fussing with it) - so certainly, I have baked cakes on every level possible. And while I'm sometimes forgetful about my oven racks, I *am* picky about quality so I certainly know and care if anything goes wrong.
My feeling is, only you can fully test and know your own oven - so if you've found that it really does heat unevenly, you'll want to test where it's very hot and where it's not so hot and go from there. But generally, if your oven heats reasonably evenly, you likely shouldn't notice any major differences between any of the *three* "middle" racks, though to be on the safe side you might limit the options to the actual center position or the next one further from the heating element.
To answer your question a bit more succinctly, if it says "position rack in center," I do take that to mean put the rack itself in the centermost slot, not to compensate for the cake being an inch or two higher or lower.