Well, that certainly got a response, and a rather overwhelming one at that.
Whilst I have always preferred round bases, they just look so much better, I went with squares as that is what Mordheim traditionally used.
Having already rebased the Kislevites and Beastmen I will get around to doing the rest over time, I just wanted to be sure that I had not overlooked anything before doing so.
I'm a fan of round for skirmish and square for regiments. But the rise of mdf movement trays ,does somewhat dampen my logic.
I am with you there, as I will be using movement trays with the Beastmen in OGAM so that they can be 'ranked up' more easily.
Round for me too.
Mordheim bases always looked off to me, all these figures at odd angles with their bases digging into each other, trying to figure out if corner touching a corner counts as base contact...
plus 25mm rounds will give you more room to play with the basing and frame the figures. 
I agree that square ruins the aesthetic.
And I do like to mess about and make a base.

HOORAY! KRAPPEFORTT RETURNS! The new Old Beasty Boys look WONDERFUL on their LOVELY, new, round bases. OUTSTANDING WORK!
Mordheim was never specific as to the bases. They included bases that were right if you planned on using the minis in War Hammer as well as Mordheim.
I really have come to love the round bases myself. I am tempted to rebase the stuff I have on square bases; only inherent inertia is holding me back.
Thanks for the eternal encouragement, Al.
Much appreciated.
I shall rebase mine over time, so you may feel the urge over if you keep peeking...

But I cannot think af manny more soulcrushing boring tasks than to rebase a whole collection.
Maybe I am weird, but I do not mind and find it quite rewarding.

Always liking round bases for skirmish games - looks better in every way. The one exception possibly being Mordheim! Squares allow you to clearly put the model face up/down, while round bases can be more difficult depending on the pose of the model. After much deliberation, I've decided to put my new Skaven band on squares for that reason. That said, round would probably have looked better...
This is the kind of thing that I was hoping for with responses, as I had not thought of that.
The solution is quite simple, I think.
An 'arrow' on the underside of the base facing forwards would cover that, showing whether the figure is fave up or down.

It's got to be round. One thing that puzzles me when I look at "Oldhammer" blogs is that people now only seem to use the diagonal slot on a square base. I remember spending aeons cutting out the straight slot to ensure that figures would at least be facing forward when ranked up.
You are not alone with that.
I have lost count of the number of bases that I broke due to impatience....

ROUND and TRANSPARENT!!! 
Whilst I can see the advantages of that, I do like to titivate a base so I shall be tarting them up.

Btw wonderfull figures in this thread.
Cheers,
Don
Thank, Don.
Much appreciated.

Well, after all that: those slotta will be going!
