You likely won't -have- to dose if you keep up with that water change schedule, but your tank will appreciate the stability that dosing adds. It all comes down to your tank's alkalinity and calcium consumption. If it's low enough you're probably fine to handle that with water changes, but if it gets much higher than that the stability dosing offers is worth the effort.
I'm running a 40g mixed reef of mostly LPS and softies with a few small SPS frags (birdsnest and stylo) and I recently started dosing because the tank is consuming about 0.3 dKH per day. So over a seven day period my alkalinity is dropping over 2 dKH. I suspect a lot of that is being consumed by the coralline algae though so you should certainly keep that in mind even if you're just keeping LPS and softies.
Also, dosing doesn't have to be expensive. I just picked up the BRS 2-part package with the mixtures, jugs and measuring cups for less than $50 and I dose by hand each day. At my current usage rate, this should last me well over a year if not longer unless my tank's consumption increases. Sure it could get expensive if you add dosing pumps and nice dosing containers to the mix, but simple 2-part dosing by hand is dirt cheap and as I said, my tank is much happier for it.
