As the title.
If I change uExterior Cell Buffer, must I change also the uGridsToLoad? Because I set ExteriorCB on 72 but uGridsToLoad make my fps drop down when I change it.
Any suggestion?

No, you don't need to change uGridsToLoad to change uExterior Cell Buffer. Some say bumping it up as high as 72 with uGridsToLoad with the default of 5 can improve performance. You probably want to use the 4GB Launcher if you're not already.

I just wish the uGridsToLoad change didn't have the negative effect on save games when going back to a lower setting, i know its an easy fix and work-around if you do want to go back or just plain don't save it till you know you like it, but still i think it should be a setting in the game its self by default, it also makes me wonder what lasting effects other ini changes could have on save games that maybe we don't notice right off or don't know about yet.

It doesn't really do anything. Mainly placebo effect.

Refer to thread http://www.gamesas.com/index.php?/topic/1274926-ugridstoload-skyrimini-comparisons-and-explanation-default-57911/

General layout of your values will look like this, just plug in the number for where "X" is located and then do the math


so with ugrids 7

Using UGrids at 7 really does improve the distance drawing, it's much better than 5 but a good GPU is required. Nothing worse than chunks of terrain loading up near by like in the console version.

Also saving your game makes this setting tied to your savegame so reverting to "5" will make your game crash upon loading that savegame. Backups needed

Altering uGridsToLoad without changing the uExterior Cell Buffer makes the game crash, i've set grids to 7 and using fast travel resulted in CTD everytime i tried, like many users reported similar issues.

Now using grids=7 and buffer 64 gives me zero crashes for 10 hours gameplay, still rockin.

It definitely got rid of the poor looking terrain/rocks, minimal fps hit for me.

They are directly related. DHJudas is spot on. If you change one, you should change the other.