By wishbones do you mean the semi trailing arms ?, if so no they are plenty strong enough.
Some say that you need to strengthen the tower tops/area, where the bolt that secures the top of the shock to the body is, I never have myself and despite having towed like this never had any problem.
I would defo recommend a strut brace here though, ideally triangulated from the tower area to the centre of the boot floor under the strut brace, but even a staright strut brace would be a wise move as the suspension movement will try to impart a force pushing the towers towards each other.
tabetha