Whether Sickbay is on Deck 7 or Deck 5 depends on how many decks you plan to include in the saucer overall. It makes sense that the facility be in "the best protected part of the ship" (Kirk, EOT) which would be in the centre of the saucer.
Using FJ's plans of 8' ceilings you can just about squeeze 11 decks into the saucer, making Deck 7 fit this location nicely.
However, we are consistently seeing ceiling heights much higher than that in the show - 9' or even 10' is not uncommon. For that (and allowing for a certain amount of deck thickness) then the centre of the saucer would be around Deck 5.
Incidentally, the Jefferies cutaway
also shows Deck 5 in this location (not that it was ever seen onscreen)