The main reason I invaded the Saxons and the other barbarian factions to the west of the Franks' starting position is so that I could really build up my economy and armies before taking on the Romans. You need A LOT of money to convert buildings not of your culture to your own, so if you're planning on invading and occupying cities belonging to a different culture you need a sizable reserve of cash to convert a settlement; 20-25,000 at least.

Also, ignoring the Romans and letting the other barbarian factions have at them saves you from both having to fight the Romans (whose units are quite formidable) and from having to pay the conversion costs of buildings since your "unwitting pawns" will convert all the buildings for you. And if you're feeling really nasty, it also weakens the barbarian factions militarily and economically so you could also attack them after they've bloodied their armies or emptied their treasury.