Once you can go for the kill, yes. First I usually try to soften up the defenses, to knock the FP down to 2-3 units. Since if the battle goes bad, you can move in more regular troops to maintain your 5 combat dice.
One mistake I did make is with Grond, don't have 5 defenders and a leader in seige when you play it... you could kill off your 10 army and the nazguls and witch king... if you roll poorly and they hit 10 times over the 3 rounds... doh...
Combat cards is the key to taking strongholds (imo). Onslaught, great host and relentless asault are 3 good standard combat cards to help you take down a stronghold. In my experience, it's important to kill the stronghold using only 1 die, to prevent the FP player to use eventcards to muster extra units.
LIIT WOTR fan - We are looking for ppl to play against IRL or via webcam - just for fun - PM/reply if you are interested -