If you want a guaranteed spot by the pool, you'll have to reserve a cabana. Otherwise, the provided pool chairs are first-come, first-served. Honestly, if it's just 2 people, they shouldn't have a ton of trouble finding chairs any time of the day, though possibly not in the shade. If it's a family of four the 3rd week of June...well, that might be trickier. Let's just say that I wouldn't mosey down at 11am and expect to find 4 chairs together.
That being said, it's not really a huge deal unless you absolutely HAVE to sit in a shaded spot. Otherwise, finding a seat by the pool is one of the last things you should concern yourself with because it will be what it will be. Of course, having your own beach chair is a nice alternative, too. There's aways room on the beach (though not necessarily in the shade).
FYI, we had no trouble finding poolside chairs in low season. Even so, we still rented beach chairs for the week from Baby Rentals because we love to beach hop. We love packing a picnic lunch and exploring the island. And I honestly can't tell you what beach was our favorite because they were all outstanding: Ho'okipa Beach (watching the surfers and windsurfers), Makena Beach State Park (calm and beautiful and lots of shade), or Honolua Bay (outstanding snorkeling in the summer).
The Tommy Bahama chairs from Maui Baby were very reasonably priced, and they had nice carrying straps, cup holders, and even a soft-sided cooler to bring your lunch. I cannot imagine a better deal on the island. One week's rental was about the same as a one DAY rental from WKORV.