I think it depends on the rest of your applications. If you have lots of research experience and everything else is strong, if your GPA is lowered by Bs in grad courses, the committee will probably take a closer look and realise you took the more difficult grad courses (or perhaps you would want to mention this -- the fact that you took lots of grad level courses, not the lower grades -- in your SOP). If this is the strongest part of your application, then you really want to emphasize it.
Overall, I don't think it could really hurt you to take grad level courses instead of undergrad ones. However, if you are taking the grad level course (e.g. grad QM) then I am assuming this means you already took all of undergrad QM before taking the grad QM? So even if you say, get a B+ in grad QM but got A's or better in undergrad QM, then the committee would be happy -- your high undergrad marks allow the grad courses to be viewed as a "bonus".
But, based on your last post, it might sound like you are considering an undergrad version vs. graduate level version, which makes me guess that you might be talking about cross-listed courses (i.e. same class for graduate and undergraduates but usually the grad students have a few extra assignments or a project etc.). Or maybe you are talking about the grad courses meant for new grad students who did not take topic X in their undergrad, rather than a grad course that builds on undergrad knowledge of topic X.
In these cases, it's my opinion that taking the grad level version would still be good -- it will show the committee that you like to challenge yourself. However, since GPA is still a factor of getting into schools, and that you don't have an undergrad senior level grade in this topic to "back you up", then be sure that you are not biting off more than you can chew! Setting priorities and knowing your limits is a very important part of being a graduate student, so if you went all out and took all graduate courses and got all B-'s, that's not going to be very good. That said, your application isn't going to reflect the marks for courses you take next semester anyways, so just score as well as you can in this semester! In addition, QFT is something not covered in standard undergrad physics, so that's a grade you don't have to worry about as much, since it's going to be "bonus".