I'm not too experienced at Epic yet, but I've found larger bases can cause difficulties with transport vehicles and finding cover.
I base my models on Warmaster bases (2cmx4cm) rather than the strip bases (1cmx4cm), and it's the convention within the group I play with that an entire stand must be within 5cm of the transport it is deploying from. For vehicles with a large transport capacity (eg. Thunderhawks, Landas and Vampires) getting all 8+ stands to stay completely within a 5cm bubble is almost impossible (I always take some jump infantry, as their 15cm disembarking really helps).
With cover most opponents can fit in twice as many stands as I can. That's both good and bad though- a bit of artillery often makes them less enthusiastic to take advantage of this.
I've seen armies mounted on round bases which look really good, but round bases really hamper combat armies. Trying to get two round-based units to touch each enemy stand is usually not possible, so expect to rely on Fire Fight rolls. It can also cause problems with transports, as the models must touch the transport to embark and this can make things really tight around high transport capacity vehicles.
Some people I know base their armies on the 2cmx2cm squares so that their army is valid for both Epic: Armageddon and Epic 40K.
I think one reason for the strip bases is simply that most 'current' epic models come with them. All of the main plastic boxes contain/ed 1x4 strip bases.