I have 2007 350
My steering does self center, but it does so very slowly at low (car park) speeds, so I do need to steer it straight when coming out of very slow corners.
Once past about 15-20mph then it feels fine, but I wouldnt use self centering at this speed.
Directional stability on the motorway is fine, and i could drive (say if eating a pie and using a mobile at the same time) with no hands on the steering wheel and feel safe (its my wife who doesnt feel safe
when I do this)
I have just had a four wheel alignment done, and it is in specification in all aspects.
Also the steering is heavy at low speeds, but fine at higher speeds
I find it odd compared to a modern family car, but just put it down to a characteristic, and maybe it adds greater "feel" or neutral handling.
If you are in doubt, ask to get a four wheel alignment check done.