Installing a hot tub that sits "flush" with the deck may seem like an attractive option, but the reality of climbing in and out of your hot tub when it's 100% sunken into a deck is not so fun. Most people would rather sit on the edge of the spa and spin in, making it a more comfortable transition into the warm water. I recommend leaving 12-18" above the deck surface. The other big thing you need to think of is accessibility to the equipment compartment for basic maintenance. A flush mount spa is typically going to harder to maintain and if you ever need service work done, will be more costly. The picture here actually shows that these owners left two sides open for easy access and from the upper deck, it looks like a disappearing edge!

the salesman was right. we've had a hot tub since 1995 and built the deck around it. halfway is really easier than flush or even totally on top of the deck. you will appreciate it as you grow older, bad knees and all.