There are no rules but I really like using Sample Manager by Audiofile Engineering to change the sample rate of stereo files when needed before working in Wavelab. It’s a pretty affordable program and it uses iZotope’s Sample Rate Converter which is known by many to be a good sample rate converter.
Sample Manager is also a great yet simple batch converter overall. Sample Manager is a little dated as they are working on an update for it with a new name and approach.
You could also use iZotope RX4 standalone app to convert the sample rate of files which likely has a more up to date version of their Sample Rate Conversion but RX4 is also more expensive because it has many other great audio repair tools and features. It’s worth the money if you are in the market for excellent noise reduction and other repair tools.
I don’t have any comments on the sound of Crystal Resampler in Wavelab because I personally don’t like to run any SRC live on file renders. I like to have the montage built at the final intended sample rate.
So, my opinion would be to bounce your Pro Tools mix at 24-bit/48k. Convert to 24-bit/44.1k using Sample Manager or iZotope RX4, then do your Wavelab work.
There are times when I have to create two versions of the same montage, one at 44.1k and then again at the original sample rate like 48k or 96k because the client also wants masters at that resolution.
I really dislike the idea of running Wavelab Crystal Resampler live in a montage because of extra rendering time and then double checking each render for an error or glitch due to the SRC.