Well, I am sure that this largely depends if you are playing AIs or humans and how the game itself is shaping up.
In my current AAR (vs. AIs) due to my fairly large army and passive AIs (likely due to my large army) I don't bother building forts at all and just steamroll the AIs, trying to siege their home fortress as soon as possible.
When I am afraid of counterattacks I will build some province defenses (PDs).
Works fairly good till now
Vs. better AI or humans or simply by being later in the game (tad below 50 with this AAR) with more access to flying attacks into my rear areas I am fairly sure that such a tactic would be suicide since my rear area is virtually undefended and my supply lines are fairly long.
So as most of the time with Dom the right answer seems to be "it depends" and be prepared to change your strategy during the course of the game.
Mainly the availability of certain magic spells to my enemies would have a fairly large impact on my defensive behaviour (and thus my offense).
I would try to guess the time of my enemies getting abilities which would endanger my rear areas and beef up my defenses about 5 turns before that time.
Till this time I would try to grab land as fast as possible without blocking the AIs from areas to expand into if I can prevent it since this might force me into war with them which I am most likely not prepared for at this time.
CharonJr