Plants that are allowed to rest absorb light more efficiently. This means that keeping your lights on 24/7 might make your plants grow a little faster, but it will cost you 20% in energy bills for way less than 20% increase in growth. In the end, the most efficient growth/energy used lighting schedule happens to be 18/6. CFL bulbs are small and very efficient and the minuscule increase in your energy bill might not matter in the long run. In fact, with a small enough grow and over a short enough time frame, you might save money on not having to buy a timer.
TLDR; 24 hour schedule will have your plants grow the fastest, but at the cost of a higher energy bill.