You are almost certainly right, m0j0. I didn't notice the angled headstock.

If the strings are pulled into the grooves at both ends of the nut, then you won't need a retainer bar, but if not (e.g. if the string post holes are particularly high and you don't wind the strings a long way down the posts), then getting one is recommended. The truss rod cover shouldn't get in the way as the bars normally have posts between the low E/A and B/high E strings, but the truss rod cover's shape can be modified to suit if necessary.

Ibanezes vary depending on the neck shape/headstock fitted. My RG08 definitely has a flat headstock and a string retainer. Some S-series have a flat headstock (my mate's new S6570SK Prestige has one) but I know most of the S-series models have angled headstocks. The current RG range tend to be more mixed and some have flat and some angled headstock. Plus they only need a string retainer bar if they have a locking nut and a flat headstock.