It mostly depends on what your end goals are. Starting earlier than day seven can be fine, or even better, for SOME builds. It just requires alot more attention and planning then most people care to put into it. If you intend to start earlier, just make sure when you create, you do not put all of your secondary points into an attribute you are unlocking in the multi. For instance an elusive back, if you cap speed first and unlock Agility, Strength, Carrying , then put almost all your points into stamina or vision, or some pertinent stat that you won't be getting to anytime in the first season.
The biggest reason to start earlier , is to avoid using BT training any other way, once your primary is at the point you need it (alot of my builds start around day 2, once main is at 52.5 and 90+ %, so i can cap it to 61.5 at level 4, roll it over as part of my multi, and be able to reach 68.5 by lvl 8 (with plenty extra SP)). If you only get so much BT to lose training something other then light, i will usually chose that way to be multi as much as possible, since its by far the best train/sp/bt ratio in the game. Just try to use Intense as little as you can to reach your goals, cause it hurts your BT the most, and is rarely worth it.