Commentaries should not be out alone. It’s much safer if they travel in groups of at least two.
Let’s say that I have done my work in the text and want to interact with a commentary. I turn my chair and reach toward the shelf. I tend to always grab at least two. (Sometimes they are on the computer, but same principle applies.) Why? There are a couple of reasons:
1. Because different commentaries offer different strengths. So I might choose to look at a single-volume commentary that will give me quick access to background matters and quick flowing summary of the passage. But I also would benefit from looking closely at a key section in the passage, which I would get from a more technical exegetical commentary. And I might go somewhere else again for slightly expanded applicational nudges.
So for a slightly overworked example, if I were working on a passage like Hebrews 11:13-16, I might find it helpful to get the overview of a single volume commentary like the Bible Knowledge Commentary or New Bible Commentary. I might get slightly more coverage, but still not probing the text technically, from Expositors Bible Commentary or the Bible Speaks Today volume. Then for technical wrestling with the text, I might grab for Ellingworth’s NIGTC, or Lane’s WBC, or Bruce in the NICNT series. (Actually with Hebrews, I’d also be checking Koester’s ABC and maybe Attridge’s Hermeneia volume.) Then there is Guthrie’s very good NIV Application Commentary too. That’s quite the gang of scholars! And I haven’t mentioned older ones like Owen or Calvin.
2. Because one voice tends to be more compelling than two in dialogue. Ok, it is a bit unrealistic for most of us to have access to a library selection like that one, but we must be careful not to rely on a single voice. Some people love MacArthur, or McGee, or Tom Wright, etc. Even without raising concerns about single voice complete Bible series, I do want to raise concerns about just listening to one voice in a single book. If you only read one, then they will probably seem compelling to you (or easily dismissed by your superior knowledge). That is the main reason I always grab two from the shelf. Compare and contrast, and you will reap more than double the benefit (as long as your collection isn’t completely mono-vocal in that it is all from the same theological camp).
Tomorrow I want to point out that commentary and preaching are not the same!
Biblical Preaching 
I find that along the same lines as tip #2 that I need to delay my use of commentaries until I’ve studied the passage a significant amount on my own. If I consult a commentary too soon it tends to lay down railroad tracks through the passage and I can’t break my mind away from reading it in that way. Consulting multiple commentaries helps in that regard but delaying their use helps me even more.
I have a slightly contrarian view on this, for what it is worth. I used to think this way, but I found that sermons I’ve heard or even preached in the past, or just the way I’ve studied it out previously, has the same effect. The railroad tracks are there anyway.
Sometimes, reading multiple commentaries helps to break those railroad tracks. So these days, I use commentaries earlier in the process than I used to, to guard against missing anything I should be considering.
Sometimes, I’ll read a commentary, and the flow of the passage will become so obvious and yet I missed it because of preconceived notions. So then I have to go back and rework everything again, if I’ve delayed reading the commentary.