Is it cost effective to use your power nodes to unlock doors?

It depends on the difficulty setting really. If you're playing on Normal, you generally would be better off using your nodes on upgrades as supplies aren't really an issue. On Hardcore difficulty, you will want every medkit and bullet you can find, so opening every door is the way to go. Nodes will generally not save your life whereas bullets and medpacks will. In my experience it's wise to open every door regardless of whats inside, as it's always going to be useful in hardcore mode, arguably more so than an extra upgrade or two. For other difficulty settings, strategy is not nearly as important, and you can pick and choose depending on what upgrades you want specifically.

At least according to the game itself, the contents of the rooms accessed with power nodes are randomized. I haven't taken the time to sit down and figure out though if they are all worth opening. I know I did find at least one ruby board (worth 10,000 credits alone) in one of the rooms later on.

Some of the rooms also have schematics in them which will allow you access to weapons and suits early. I've heard that in Dead Space 2 if you don't find the Advanced Suite schematic early, it will still be available to you later on in the game, but that early advantage may be considered making it worth it.

I always open the doors. I would estimate that 3/5 times they will pay for themselves in supplies. I once found three power nodes in one room. They also often contain blueprints, so if you have a favorite weapon or suit, you're a lot more likely to miss it if you don't open doors. I'd say it's always worth it.