Exactly, whether truss rod adjustment is necessary depends on several things, including your preferred action. Higher actions are more tolerable of minor tension changes where super low action can require frequent adjustments. Also depends on how stable the neck is. For instance, I've got a few necks that bend really easily and would require adjustment, and a few others that are solid as a rock and never need any changes. Quarter-sawn, flat-sawn, multi-piece construction plays a big part in that and one is not better than any other, just different.
Ultimately, changing from one gauge to the next, like 10s to 9s, is roughly the equivalent of changing from standard tuning to Eb.
As pointed out, it's a good idea to check the relief before and after, if for no other reason than to know where you started from and can adjust to get back there. And it'll take a day or so for the neck to settle in to the change in tension.
Trying out a new gauge isn't going to hurt anything and at least you'll know if you like it or not. Just remember, when adjusting the truss rod, don't turn more than about a quarter turn at once without rechecking. It shouldn't take too much effort to turn, so don't force it. Finally, some vintage style truss rods, only tighten, so if you need to loosen, it will take longer for the neck to settle. Modern truss-rods can adjust either way. Don't over adjust and recheck after a day.