Nice models, Steve!
During modeling, it is often very handy to model parts as separate Objects (they can use different Modifiers, just to name 1 advantage).
When the model is finished, it is up to you, mostly. In the case of your snowman,for instance, I’d leave at least the hat as a separate (Parented) Object (so you can easily change its position and orientation on the head and Duplicate it if you want and so on), I’d Join the crate parts into 1 Object (unless you want to also show a version, where a part is lying next to the main crate) and in almost all cases, I’d also Join a keyboard into 1 Object (or maybe 2), even though all the keys are separate in real life (think of the ‘nightmare’ if you’d have to name each key uniquely).
But you’d have to consider each situation individually. When in doubt, it might be better to leave them as separate Objects; it is always easier to Join them later on, then to Separate Parts afterwards.
