I steam block acrylics (and everything or I find excuses not to block), but I don't think I would do that with random types in one piece. They don't all react to the high heat the same and it would probably be impossible to do a test swatch.
If they're all machine wash/dry that is all I would do. The first time either take it out damp and lay it flat or take it out warm (don't run it through the cool down cycle) and lay it flat.
You probably should test swatch the mystery skeins, they may not take hot water or hot drying.
I didn't believe the label on a nylon blend and my disbelief turned a 2 year old sweater into a 1 year old sweater (luckily I had sweaters planned for both, but the 1yo probably won't get much use out of the now thick sweater living in SC).
If you have a single yarn acrylic piece you can change the shape with steam and it does hold. Steam also does what some term "kills" the acrylic. But IMO that makes the acrylic drape like more expensive natural fibers rather than "hard" acrylic.
The importance depends on what you desire (different blocking has different effects) and the piece (steam blocking an acrylic blanket with curled edges will calm it down a lot faster than washing and drying).
When I had my recently finished cardigan pinned together with the body blocked and the arms not blocked the unblocked arms were not something I would want to wear while the body felt good.