If you don't have any +healing gear, then the mana-to-health ratio for each heal is basically the same. In this case you're generally better off using big heals every now and then rather than small heals continuously, because big heal - break - big heal etc etc allows you to regen mana outside of 5 sec rule.
Since your current biggest heal usually covers +/- half of tank's hp, this changes when dungeons become a bit more hectic. In BRD you don't want to wait till the tank has 40% health before healing, that's suicidal. Of course you won't have 0 +healing forever, you should have around +100 from damage gear alone around lvl 50-55. So some downranking becomes viable. Once you start downranking, you'll probably want to use heal r3 until you accumulate enough gear to make heal r2 useful.
Don't shield people unless you fucked up and they're about to die. That's about the most mana inefficient spell you have. Same for flash heal, only in emergencies. Renew is okish if you have 3/3 improved, otherwise waste of mana also. Really, if your tank isn't a complete fuckwit running around in lvl 15 greens, most dungeons up to BRD require nothing except an occasional lazy heal. Get a good wand, you'll improve the group's dps significantly