I had some grapewood in my tanks for a few years, and did not have a problem with rot. I prepped it by baking it for an hour at 300F, then submerging it in a bucket for about a month - which is about how long it took to become water logged and sink (the baking surely removed all the water). I changed the water a lot to pull the tannis out.
I ended up pulling the wood out as it seemed to be messing with my ph/kh when I started dosing CO2 for plants. Otherwise, it was fine.