Do i have to actually paint the relevant sacred spawnings on the units to use it? Can i simply decalre which units use which spawning at the begining of the game and add up the points?

I guess it depends what kind of game you're playing, most friends and possibly non-competitive games wont be too bothered as long as you point out to them at the start which units have which spawnings. More competitive games and tournaments may enforce the WYSIWYG rule and state that you must show which spawnings each unit has with a relevant paint scheme.

Usually the rules about showing WYSIWYG( what you see is what you get) is relatively laxed as it would be annoying to repaint your minis for different matches. If your opponent really wants to know though, what you can do is make special banners for different spawnings and then display them on/over the unit somehow to let them know. You could even replace the standard bearer if you have one to display it.

Very true... didn't think about repainting every time you changed... someone like me would have a nightmare haha.

Yeah, Id probably just give up and dip them all in paint if I had too. Much quicker, but a very bad idea. The banner thing seems the best way to represent it if they get strickt on WYSIWYG rules, just use the symbols in the army book on it.

I never painted nor will I paint sacred spawnings on my units. Also, I had never any opponent complaining in any way and even GW cant decalre WYSIWYG for that. They would scare of any Lizzie player whouses spawnings. Also, nearly every Lizzie player has a different Painting scheme, so there would always be big confusion if GW would explain at any point thta WYSIWYG would suit for Saurus spawnings.

I can see how, from the opponents perspective, it could get annoying though. Not that im saying you need to paint them differently, but maybe just have some sort of a marker to distinguish which ones have which spawnings. Imagine playing a game and having a bunch of aquatic saurus' that you didn't know had the spawning charging across a lake and flanking you or something.

I paint all of my units a bit differently. The row of larger scales on the back of the saurus is red for one unit, yellow for another, and blue for the third unit. Just write those colors on your armylist, and there won't be any confusion..

Is it really needed though?
I barely use spawnings except for on caracters and in sacred hosts, and in those cases marking them isn't really needed :)

As far as I know, you dont even have to tell your opponent wen your unit has a spawning, you just have to have it on your roster sheet (obviously). Thats at least what the only lizzie playing red shirt in my local GW says.
Right now Im playing to repaint some old stuff of mine and sculpt thicker scales for my first unit of saurus to represent mark of quetzl as I use it very much.
Maybe Im gonna add a small unit with mark of sotek for my red host of tehenhauin some day.

That like saying you don't have to say which mark your chaos army has.

The only thing players can keep secret is magic items. Spawnings have to be anounced.

I think what he might of meant (correct me if I'm wrong Huanloq) is that you'd trade your roster sheet to your opponent for him to see, so it's kinda announced that way. I guess you could have which unit had which spawning on your roster sheet too, but it might be hard to indicate.

Especially with my older Sauri...
I got blue, green and even an orange saurus in there.

Usually I'm pretty limiting my sacred spawning to "I'm using the Sacred Host of ...".
And to characters I got will probably have their spawnings announced. (except for itzl, itzl is just plain obvious)