Not sure I understand.  If you buy gold or experience early on in the citadel it's the gift that keeps on giving to your entire team.  With more gold you can more easily make up enough money to buy that special breastplate you always wanted.

The thing is, you must balance citadel upgrades with Demigod upgrades.  Often you won't need to upgrade your citadel until you're level 9 or so.  For instance, if you get priests too early then suffice it to say that your opponents will often welcome this decision; that is, early priests can sometimes "feed" your enemies.  That is, if you don't have good enough control of the map, they can creep grind and gain more levels (since priests are just another thing to kill for EXP). 

Basically, you have to choose your upgrades wisely and timely.  I am aware that you weren't implying that you don't EVER upgrade your Demigod, but this can be just as important.  You don't want to get utterly smashed in a 1v1 versus an opponent of the same level or even lower.

Both

I have been waiting till about level 5 to bother upgrading the citedal. Thinking back i'm not so sure i'm going about it the best way, but so far it has been working for me.

It's a continual learning process.

I agree though that a decent balance is needed with a slight favor to your Demigod.

Getting EXP int he citadel early though is crucial.

I try to be balanced about it.

Eh, I don't normally bother with citadel upgrades. The most I'll do is invest and get all the reinforcements, but only at the end of the game if I'm trying to destroy the enemy citadel... which doesn't happen often.

I spned maybe half my money on the citadel.  My teammates always buy the wrong upgrades...

Both. At the beginning of the match usually just myself, I'll spend the 1k on myself, then the rest will generally go to citadel for a bit.

ranged demigods don't deal damage and support demigods don't need mana and health?

I usualy try to split it up evenly. Right off the bat I tend to buy the building health/healing citadel upgrade so my towers can survive a beating better and the unit health/durability upgrade just to give my guys an edge over the enemies when I'm not there to help them out. Also makes them a bit more annoying to enemy Demigods.

It all depends on if your Demigod can stand toe to toe with the enemy demigods or not. If you can usually win 1 on 1 fights, or almost always slaughter the enemy team when staying with your teammates and working together, then it makes good financial sense to upgrade your citadel while you still have the upper hand at the demigod level.

If you don't have the upperhand at the demigod level, upgrading your citadel will only help if it stops them from being able to push through your line. - I.e. +splash damage and +regen, you'll probably want to get that extra item to push you over the top edge wise, so you can start getting those sweet gold bonuses for killing enemy demigods.

As I often play Regulus, I am usually spending more on myself than in the Citadel. But crucial upgrades like gold, exp, and +regen are absolutely critical. They will win or lose a game for certain.
-Aleckor

Hmm, now I wonder why that might be.