Why do you place your resource cards upside down? My guess is so that they aren't somehow confused with cards in play.
That, and it's easier to see what resources you have at a glance, I'd wager. But since resources are usually covered by a domain card of some type (from unused cards all the way to actual domains) there shouldn't be too much trouble telling what is in play and what is not.
I put mine off to one side as I like to have a larger play area than what the rules recommend.
I have only found 2 instances where knowing what cards are resourced has made a difference. It is a rare occurrence but can come in handy. I generally dont bother as it does slow the game down so unless I really need to count cards and probabilities, I will refrain.
I sometimes announce what cards I am resourcing so if my opponent cares he has some indicator other than having to dig through mine for some idea. Not sure if it is appreciated or not, but it is a habit I should probably get into more for sportsmanship's sake.
I will say I have never seen an opponent request or even care what is resourced by me though.