From what I've read so far (sorry, no play examples to draw from yet) I don't think I'd go with tag compels. I just re-read the pages on how compels work and it sounds like if you want to make that sort of call as a player against an NPC you're intended to pay for it.
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There is an economy to fate point use, and I wish it was a bit more clearly stated.  Here is my take on it.

Effect                                   Cost
Narrative control that benefits the player = Free Die roll, Setup action or 1 Fate
Give yourself a re-roll or a +2 = Setup Action or 1 Fate
Narrative control that Limits the actions of an enemy = 1 Fate Plus the enemy gets the Fate if accepted or looses a fate point if declined.

The fate point is the most expensive way to accomplish the effects listed.
It appears that the game considers player narrative control to be the least expensive effect, because it can be put into effect with a free die roll.
Next costly is giving yourself a bonus or a re-roll, this costs either a tag (which in effect is costing you an exchange and only on a successful die roll).
Finally a compel is the most costly because it will always cost you 1 fate and it may not always work as intended (but it will always do something).

p.s.
I only bring this up, because it helps me understand how to arbitrate invokes for effects and compels, because it indicates that compels are considered more costly than declarations and thus any invokes for effects that border on the strength of a compel should be carefully considered.

After rereading the rules for invoking for effect, compels, and all the examples in the book I can find. I agree that having V fall in the stairs is a compel and not an invoke for effect. It makes PC's using narrative control to short circuit much more costly which I think it more beneficial.

Thanks for clarifying all this folks, especially before our game tonight.

JesterOC