It is often a gamble when we make this type decision so I look at the odds. One is how much do I trust the person, company or country that made the item. Do they have a good reputation for quality or do they sometimes ship out shoddy materials? Then I balance how much trouble it will be if the item fails versus how much trouble it might be to protect it.
Some points I've learned? The probes are cheap but I have to shop to find them and wait to get them which leaves my unit dead for a time. My first three which were supposed to be waterproof, got wet and failed within three weeks to 6 months. I know they got wet because I took them apart and found water. I currently have 7 that have been run in covers and none have failed. That is a very small sample and doesn't really mean too much but it does give me some input.
I find I can pick up the great big long straws from fast food, seal it and stick the probe in very quick and easy. The straws give the probe wiring just enough stiffness to make it easy to place them. Silicone, suction cup or just wedged behind the filter intake tubing, all seem easy. Keep in mind that you want to keep the straw top above the water? You want to seal the water OUT, not IN!
For me, the ease of lighting the straw and then clamping the end shut to seal it versus the trouble of shopping, waiting, and then replacing the probes look like a pretty simple decision.
For placement, I can think of no better place to get real circulation as well as be hidden than the back side of the filter intake.