Both steel and plastic reels have zillions of successful users, so there is only personal preference at work in the choice. Every now and then, plastic reels can be soaked, scrubbed with a nail brush and dried because if there is a consistent problem it is likely to be due to some crud dried on somewhere - prevention is better than cure. Plastic reels need to be bone dry and if steel reels are used wet (ie. mostly dry but with an occasional drip) then the film might want a pre-soak to avoid wet patches of emulsion showing up overdeveloped when the dev time is short.

A steel reel can theoretically be dunked in almost boiling water to heat up, then rely on the absorbed heat to dry the reel off for re-use -- on the other hand, plastic reels are much cheaper than steel (so you can have two rather than one), more robust (no invisible bending when dropped, meaning also that second-hand ones are more reliable) and dry out quickly in film drying cabinets. It's choosing between the swings and the roundabouts.

In low humidity locations, will a humidifier in the (pseudo-)darkroom help with the curling of the emulsions before loading? The O.P. was talking about curl but I'm not clear which direction he meant, along the film where it has been rolled up or across the film due to contraction of the emulsion in a dry environment.

For hanging the processed film to dry I use the well known tip of drying in the shower, after steaming up the air to raise the humidity. When it eventually dries out the film has much less curl like this, in both directions. If necessary, the neg sheets then go under some large photo-books. Unfortunately there seems to be no photo-quality osmosis effect from the Salgado book to my negs - maybe I should use a paperback edition instead of hardback?