Using a Leviton GFCI self grounding outlet on 1950s house with metal boxes that might or might not be grounded
No no no. A "self-grounding clip" is nothing important as far as your concerns about grounding.  It doesn't help at all, and it doesn't really hinder either. 

The purpose of a "self-grounding clip" is to resolve some minutae in Code. I assume you've removed a receptacle or switch at some point in your life... you know how there are 2 screws, top and bottom? Ever think "Hey, these screws are metal, and so is this yoke. Is that a valid grounding path?" Code says "Yes" for switches (since they don't need much) and "no" for receptacles (because the yoke-to-screw-head contact is pretty flaky, especially when you're pushing a plug in). Self-grounding receps have an extra spring clip to make that yoke-mounting screw contact reliable enough. That is all.

As such, "self-grounding" doesn't make a hill-of-beans difference to whether the boxes are grounded. 

If a ground is isolated, it doesn't matter either way - though it can be valuable to retrofit ground to that location.   

The only thing of concern is if you have a group of outlets whose wires are connected to each other yet are not grounded to the panel and grounding system.  In that case, they aren't grounded, and worse, will share ground faults - if one of them has a ground fault, it will place 120V on all their grounds!  So you could get nailed touching your PC, when actually it's a heater that's ground faulting! 


  I was told in order to be up to code all un-grounded 3 prong outlets need to switched to GCFI outlets.


That's the simpleton version.    The real version is fitting 3-prong outlets in grandfathered 2-prong sites require ground, or, may instead be GFCI protected.  The GFCI protection doesn't need to be at the receptacle. It can be anywhere, e.g. fed from the LOAD terminals of a GFCI breaker or other device upline.  Also, footnote, outlets thus protected need a sticker saying so. 

Perfect world, you only need one GFCI per circuit if you are smart and the wiring cooperates.  If you're fitting 2+ GFCIs per circuit, you may be better off fitting GFCI/AFCI breakers - it's much easier to install, and you get better protection including from most "old wiring" problems. If you're interested, pop off your panel deadfront and shoot us a well-lit picture (yeah I know, hard to do with the main breaker off lol) and ask a question about how to GFCI it.  We know what to look for. 

If you are choosing "One GFCI recep at every receptacle", then do not use the LOAD terminals at all. The are ONLY for extending protection to downlines. The LINE terminals are designed to take 2 wires, if you read the instructions carefully.