Well, sort of. In CtP1, many of your cities will eventually get big enough to work all tiles, at which point you *can* put tile imps on all tiles.
In CtP2 placing tile imps on every square is only wise if you have more than enough PW available. Usually however, you'll find that you have a lot more tiles to spend PW on than you can afford to. In that case it would be best to first/only build tile imps where you get the most benefit. It's usually not a smart move to build tile imps in the 2nd ring of a size 7 city if there are other cities who still have room in the first ring. But of course, once all inner rings are filled, you could start filling the outer rings...
On the other hand, if a lot of cities are still only working a small portion of the outer ring, it might be wiser to just lower your PW tax and spend more shields on city production rather than on PW. Saves you the trouble from protecting your extra tile imps from being pillaged as well. Then again, in the future you will probably be able to reap the advantages of your present investments, so you may want to continue building more tile imps after all. So should you build more tile imps or not? That a choice you'll have to make... So there's a bit more strategy involved in the CtP2 system