If you stabilize before casting, the blank will turn with a better, though not perfect, consistency as you pass from wood to resin on the lathe. This is especially true for many burls as the wood tends to be light with inconsistent grain ie burl, inclusions, etc.
However, it is entirely possible to cast resin with the burl not being stabilized. Some hard, solid burls will have no issues and more common burls such as buckeye will need to be turned with sharp tools and a light touch to prevent cutting to deeply into the wood as you pass between it and the resin. Keep in mind that while the casting resin will fill accessible voids, it will not penetrate like the stabilizing resins do.
I would tend to stabilize first as my preference.
