Mandatory Substrate?

[Minkness]

Sorry about asking so many substrate questions, but I'm a newb and so far this place is full of great info and friendly people willing to share their knowledge and experience. =)

So, my question is, do MBKs have to have a substrate at all? I ask because I currently have crestie and leo geckos and substrate is different for each one. What I have settled on is more of just a tank liner. I use antibacterial shelf liner for all of my critters except 2. I just put my AFT on eco earth because she requires higher humidity, and one crestie has simple, untreated potting soil (it was my first viv and I was going to attempt live plants....just haven't dumped the dirt out yet lol).

I do like the more natural look of eco earth or reptibark, or even sand so long as it's not a harm to the animal. However, my leos only require a moist hide, and I was wondering if having the liner and a moist hide would be good for my baby MBK as well.

Note: To help with humidity I do spray everyone down 2-3 times a week and keep moist hides moist. =)