I would put it on the "dark side" of the vertical pipe coming out of the pump.
- As much out of the sun as possible.
- Closest to the pool.
You'll get an accurate reading that way, slightly sooner, instead of waiting for all the water to purge out of the filter and pad plumbing. I sometimes would turn on my system just to get pool temp, but I would have to wait a bit until all the water that had been sitting there warming up in the sun cleared the pad. The closer to the pool, the sooner the temp will read what's in the pool. It's a very minor reason, but that's the only one I could come up with!
By "dark side" I mean in the shadow of the pipe: keep the UV off the probe. Not really for temperature-reading reasons, but for longevity reasons.
I encase every wire on my pad in wire loom, also to keep the UV off of them. I have it in my head that will make them last longer. You could put some on the probe wire, and on the SWG cable. Wherever you have a cable jacket getting subjected to sunlight.
This stuff, which you can also find at big-box stores:
I didn't install my probe, the pool guy did. I would have put it where it is now, but
under the pipe. This is an old pic, I now have an IntellipH acid injector where that check valve used to me. I put wire loom on all the IntellipH tubing, too.
I've never read here of anyone else doing this, but my stuff is in the brutal afternoon sun, so I figure it's cheap insurance. I like the way it looks, too.