YMMV, as they say on the message boards, but custom veneer has been our core business for almost 25 years so our practices are pretty well road-tested. Here goes.
The logic of making a balanced panel (veneered both sides, more on that in a moment) is that veneer is wood. As such, it will expand and contract across the grain with changes in moisture content. We see those changes seasonally, when moving environments, and as moisture leaves or enters the panel from adhesives, finishes, etc... In a balanced panel, both faces will expand or contract relatively equally; because equal force is exerted on both sides, the panel should stay relatively flat even when the MC of the faces changes. If only one side expands or contracts the panel may warp.
Sometimes people get away with veneering only one side of a panel. A very rigid panel may be able to resist the force of the expanding or contracting veneer. But wood can be strong -- think about how stone was traditionally quarried by inserting very dry wood pegs into holes in the stone, then pouring water on them; the force of the expanding wood is enough to shear huge masses of stone. You might be surprised by how thick of a panel can be warped by one-sided veneering.
By this explanation you can see that the choice of balancing veneer can be important. Often, panel mills will balance semi-custom panels with inexpensive rotary tropical veneer where the backs will not be seen. This has led many woodworkers to think that the balancing veneer is not important and that any backer will work. This is not always the case, and in fact the Architectural Woodwork Institute specifies that balancing veneer should be as similar as possible to the veneer on the show face.
We really see this with burl veneer. Because of the way burls grow, burl wood expands and contracts in all directions (there is no "across the grain" with burl). So if we balance a burl panel with longwood we are only balancing it in one direction. If the panel is able to warp, it will. For this reason, we usually use a less-expensive burl veneer to balance where backs/bottoms will not be seen. Sometimes synthetic sheets (Gatorply, Yorkite, etc...) can be used because these will also expand and contract in all directions.
As for crossbanding (applying veneer between the veneer and the panel) I don't think it's necessary in the case of burl. It is often used with crotch veneer because crotch is very brittle and has a tendency to shatter along the grain lines. In this case, a longwood crossband running perpendicular to the crotch can help restrain movement of the crotch veneer. When adhering a burl veneer to MDF you actually have a very friendly condition: both the substrate and the veneer are expanding and contracting in all directions. We have never seen problems with crotch veneer pressed directly to MDF with an appropriate adhesive.
I hope that this information is helpful to you, and good luck!
John Costin
Veneer Services Unlimited
Custom Veneered Products for the Woodworking Industry
(207) 459-7221
vsu@gwi.net
Member