I feel that it depends on the TPG. I often see run of the mill notes graded going for much more than an ungraded version of the same notes. When you get down to the real hard to find notes, though, you tend to see the grade come in handy for figuring out the worth of the note. So, if a professional grades a $5 train note, it will likely sell for around catalogue value. Whereas a fairly normal 1954 $1 Devil's Face graded by PMG as a 68 ultra gem or whatever they call is being sold for $200 when the catalogue says it's worth about $140 doesn't really make sense to me.
For the record, I prefer BCS.