It depends on the market, fares, etc. Never assume it's cheaper to book a roundtrip and/or OW separately. Honestly, check both.

There are some knowns. IME, for example, assuming the fares are available OW, trips to Canada are usually cheaper to book as one way, because Canadian HST is only charged on the ex-Canada direction of the trip when booked separately, while it's charged based on the entire amount if you book a roundtrip. That said, if the OW fares are more expensive than a RT in the first place, then obviously, that might not be the case. Personally, I haven't seen that to be the case on my trips to YYZ (2-4x/year) in many years, but I have seen it before, so it never hurts to check, IMO.