Just to add:
I saw in one of my Canon camera manuals describing the "centered needle" as a "Standard Exposure" rather than a "Correct Exposure", since "standard" describes the "18% medium" standard for reflective meters, as was discussed.
What your instructor told you would only make sense if you had a high quality Evaluative meter and were shooting a scene that "averaged" to "medium" and, since Evaluative "prioritises" the center metering area, the subject at the center or, if you use the 7D function that shifts metering the selected focus point the subject at that poing should be reasonably medium.
But if the scene averages out to lighter or darker, the meter can get messed up as was discussed.
The Evaluative meter of the 7D does in fact have a good reputation compared to older cameras, but still these things should be borne in mind and evaluated through testing.
It's quick and simple to adjust your exposure up or down as needed, but it is important that you be aware and ready to adjust either in Manual or using Exposure Compensation in Av, Tv or P modes.
But, for me personally I prefer to use Spot metering when I'm using Manual mode. This is because spot will not do the "averaging" thing but will meter a single subject, and so it is quick to pick a subject with a known tonality and adjust the exposure accordingly, and then you are good for the whole scene!
There could be a case for using Evaluative metering mode for some scenarios, especially if you consider the scene "medium" and don't want to fuss with messing around with your exposure. It may be good to use in say Av/Tv/P if you want to trust the metering and automatic settings. But in other scenarios you'll need to be ready to use Exposure Compensation and also Exposure Lock when things get thrown off.
Anyway, stick around, and grab some time to read this informative "sticky", a "Guide":
https://photography-on-the.net/forum/showthread.php?t=414088
