Ok, then that's reassuring, I was actually worried there.
Still wonder if you actually use spline36resize for resizing needs though. Considering there now is a gamma aware resizer for avisynth (
ResampleHQ), I'd suggest switching to that. But I'm not sure if it's available for 64bit avs.
Another thing I'd suggest is switching to the
dither package for debanding, since it doesn't have the gradfun2db bug and is actually a lot better quality wise and offers a lot more control (plus it's internally 16bit, and can actually produce 16bit output, which you can use in tandem with a higher bitdepth x264). I fear this one, too, is a 32bit only plugin for the time being, however there might be 64bit builds around for this one.
In terms of quality, I'd also suggest using
QTGMC for bob/deinterlace and
smoothadjust for tweaking needs (you should never tweak though, unless it's meant as an effect within the amv). These (well, smoothadjust and most actual plugins qtgmc uses) do have 64bit builds too (QTGMC's creator also tells how to use it for MultiThreading, though he also mentions it's not stable).
TL;DR: At the end of the day, it's your choice. I still do have reasons to stick with 32bit avisynth and single thread (and I can just run multiple encodes at the same time to cut down on total time, without the fear of stability and quality repercussions).