Really it just depends on what you want to do with it, and how large the unit is. I've found unit fillers are normally OK so long as they don't interfere with the front or back ranks, so if the unit is going to be 5 ranks deep, the filler could be up to 3. The bigger the filler, the harder it is to adopt new formations though. I would try brainstorming what you want the fillers to look like, and then so long as you can fit the base into your formation without too much trouble, go for it.