I play some teams who buy flag locks early, like levels 1 - 3.
I think this is a waste of money. 250 GP is important early when you're racing towards your 1500GPish item. Unless a specific flag is required for your strategy (short cooldown on TB), don't waste your money so early when gold is sparse.
A good time to start locking is when enemy buys priests. You invest 250GP to lock the middle flag in Cataract. This buys you the time to go farm the priests in another lane and earn back your gold. Flag lock allows you to hold two flags for effectively 0 cost.