If prices are set to average or below, there isn't much need for one, plus there isn't actually any way you can assign a sim as a greeter. Whereas the Host seats the customers when they have decided to eat there, the Greeter takes on the role of selling the menu.
According to the Prima guide the customers browse the menu with a sales bar like in a shop, like in shops you can help the sale along by skilful use of sales socials. Since there is no way to assign this task in restaurants, the job of selling must be done by a controllable sim. The logical choice would be the host since they're are in proximity to the browsers and they are already building the sales badge, however hosts don't automatically deviate from seating customers who've already decided that they want to dine, you have to direct them to do it. They would return to their host position afterwards and stay there until you direct them otherwise.
Once your restaurant is successful and paying it's way, you would be in a position to hire someone to become the host and have your controllable sim take on the role of Greeter and do the 'show menu...'interaction etc. As always though, if you have other household members who can help out, it won't cost anything extra.
