To be honest, it depends on whether you can transfer the PokeRUS (easy to get if you have WiFi, or can go to a local hotspot; some people like me are willing to just give it to you) and Power Items to Platinum. If you can, sure, do it at the beginning. If you can't, wait til you can get those items (and organize a time to get the PokeRUS), and then train some new ones. I assure you that getting 10+ EV per Pokemon (or 5+ if you can't get the PokeRUS) you beat will make it a lot less frustrating than getting 1 to 3 per, especially when you can fight trainers with the VS Seeker, which really speeds up things (Magikarp guy, anyone?)
EDIT: Though I would wait til post-dex either way, because it can be time consuming, and ingame isn't that hard (unless it's well into a Battle Tower streak, or something similar). Just make sure your Pokemon are level 50+ by the time you reach the E4, and you'll be fine.
As for what I'm doing different? Transferring some Naughty Charmander eggs with the moves I want, and ditching my starter (Chimchar, I dislike it, but choosing it so my rival chooses the water starter), transferring power items (don't want to get those later, if I'm transferring I might as well get it over with), a Slugma, ect. Then after I've done my transferring, I'll play the game like normal, just with Charmander and the power items, and maybe not ending up with 5/6 of my team traded. Otherwise I'll play as usual, fighting all the trainers and grinding to level 45 at Iron Island included.
...I need less time on my hands.