For fine woodworking, I understand you should let wood dry and get accustomed to your shop (or ideally wherever the finished product will live). Okay so of course that make sense for a huge slab of wood, but is that necessary (or even smart?) for gluing up cheap pine 2x4s?
Yes, not just smart but many consider it mandatory. This is partly based on the wood, the location and the shop environment of course, but in general cheap 2x material is wood that would possibly benefit most from the wait!
You can substitute for fine woodworking with in woodworking in the first sentence BTW. All types of woodworking can benefit from letting the material acclimate to the local conditions. I've seen multiple examples of what can happen when you bring some new wood into the shop and use it immediately.
Of course there are times when you can't wait – and by can't I mean can't, not I don't want to ^_^ — and the kicker is you can get away with it. But where you can wait you should1.
If I glue them up right away they probably won't be dry but they'll be straight(ish).
- They won't glue as well as they would if a little dryer.
- The final product starting out straight is absolutely no guarantee they'll stay straight. Never underestimate the power of wood that wants to move — it can crack concrete and bend metal, think what it might be able to do to other wood (or weak glue joints).
If I wait then it will likely be a lot more effort to square them back up (especially as I don't have a jointer).
To acclimate the wood and get the best outcome sticker it, with heavy weight on top. The weight can be as heavy as you can easily manage, a small stack of cinder blocks or buckets/old paint tins full of wet sand are some good alternatives if you don't have exercise weights you can use.
Then leave it for at least a couple of weeks. As is often the case in woodworking, no harm in waiting longer.
I've watched various YouTube videos where someone shows how to make their version of a simple workbench and I don't ever see anyone talking about letting the wood dry so I'm thinking that most of them don't.
I believe you're correct, they're not just not talking about it, most aren't doing it2.
Of course, this also raises another question of whether the finishing process will (or can) seal the wood in such a way that it can't dry out any further thus protecting it from warping?
Take it as a given that you can't do this. Essentially this is possible, but it's not really feasible on a furniture item. Additionally, such a coating might be very undesirable on a workbench..... think bowling alley in socks!
1 Worth pointing out that other people's success doing this has A, no bearing on whether you will have equal success, in your location with the wood you've bought, and B, is actually no predictor of their future success doing so because even the wood where they are can and will vary. The world is full of "I'd never had a problem doing this before...." stories!
2 But I'm willing to bet many of these same people are also using table saws with the guard removed, and often no splitter or riving knife fitted either, are using their jointer without push sticks, have no guard of any description on their router tables....