Overusing declarations can kind of lessen the importance of maneuvers (using your turn to create an aspect); so I think it's more than fine to limit people to one declaration OR assessment per turn. This lets you declare (guns skill to declare that the old shotgun the farmer pulled shoots high and right, which he is unaware of since he hasn't shot it in forever), OR assess (guns skill again to see if he really knows about combat, trying to uncover an aspect) in the same round that you maneuver (athletics, dive for cover), getting two aspects hopefully to use for defense... instead of three (declare, assess, maneuver).
The reason I say this is that I found my players only wanting 2 aspects, so if they can declare and assess, then they never bother to maneuver.
Also, it makes sense to really watch what should be a declaration and what should be a maneuver: Declaring cover into existence shouldn't automatically hide you (saving you a turn). It does kind of neatly let you do a maneuver that gets you 2 tags instead of one (because you declare the cover, which can be tagged, then get behind it as a maneuver, to tag that as well).
I searched all for declarations and the only comment I found was that they "take no time in game", which i think lends itself well to being a free action. Assessments seem to be occasionally part of a longer action (casing the joint), but occasionally free actions... it really depends on what is being assessed.