Lower League clubs are generally a no-no for loaning out. Well, unless the player is around 16-17 years old, where getting games could be beneficial. Generally, you're better off loaning players to clubs with facilities of Excellent and higher, ensuring that they get regular game-time. This will help their development.
But it also depends on the player. Many times, you get foreign clubs asking for loaning a player. Now, if they player will struggle to adapt to a new country, then the AI will not play him as much due to his declining performances until he gets used to it..
Ultimately, it's your call. My main, good players, I tend to keep in my squad, and give them a run in the Cup Games, especially against lower league opposition. Allow them to play in reserves and the like - depends on whether your training facilities are good enough. Also, tutoring, can't stress this enough. Try to get them to Professional personality, and they should develop well regardless..