I'll state here at the start that I played DA:O on a PlayStation rather than a PC, but I'm one of those OCD people who micro-manage fights regardless. This is what I did:
My best / favourite / canon runthrough of the game was where I was a melee specced Rogue. Dual wield, Duelist and Assassin skillsets. I never found Leliana's archery to be all that good apart from in one or two specialised fights where getting too close was a bad idea. Rogue suits my personal playstyle the best, and it means I always have someone in my party to unlock chests and have decent coercion.
Alistair was my tank and comedy sidekick. It's what he's set up for, you get him right at the start, so it just seemed a logical choice. Though I think there's mods for the PC where you can reset everyone's skills, I never had that option. Get him into massive armour as quickly as possible by piling points into STR, then I went with a mix of DEX and CON for him.
The last two spots tended to go to Wynne and Morrigan, one mage for heals and buffs, the other specced for all-out damage. Having said this, I gave both of them Paralysis, and I think both had Crushing Prison as well. One also had Mass Paralysis. Crowd control was invaluable to me - it allowed me to command the more complex fights a lot better and limit the amount of incoming damage to my team.
No-one else really got much of a look in. Dog was useless. Leilana only handy on occasion. Zev wasn't bad, but I already had the DW DPS spot, and the other tanks just weren't Alistair.
Tactics wise, I set everyone to attack Alistairs' target, so the whole party concentrated on one mob at a time. General fights, I'd let him pick his own targets, and manually adjust if there was a magic user or mini boss that needed taking out first. My mages would demolish whatever he was fighting, and I'd often manually control my rogue, getting her to take out any extra threats.
But as people have already said, there is no one 'proper way' to set up your builds. Playstyles vary - my other half loves playing a sword and board tank, but I just don't have the patience for it. I did find though, that after a few hours playing, I leanred what spells were really handy and that I ought to have gone for from the start (back to that crowd control doohickey).
Also, Walking Bomb is the most fun ever.