Kaltic makes a good point that if you are going to want some alts, you should create all the free ones you want and level them up to around Lvl 18 - 20. Then, when you add the MoM expansion, you can add 2 more alts (that you get with the MoM expansion) and level them all up to 20 so you can get the riding skill for free (I though you need to be Lvl 20 to get the riding skill for free, but it might be lower).
The most important thing is to get all the alts you will want created and leveled up to the riding skill level while you are VIP so you can permanently unlock things like the riding skill, gold cap and inventory bags for them even after you revert back to Premium.
While you also get 2 more char slots with VIP, there's no real point in creating toons with them unless you'll be staying at VIP. You won't permanently lose the toons created with those extra slots, but you won't be able to use them if you are not VIP. Then again, you can always use TPs to buy extra char slots, but again, you'll want to to have all those toons created while VIP so they get get the gold/inventory caps permanently unlocked.
If you don't care about alts, just add the MoM code around lvl 15 - 20 and when you'll have 30 days where you can get the most out of the VIP benefits.