Dogs, like humans, are creatures of habit, perhaps moreso.
The most important thing to be considered is how your dog looks at his crate. Is it a prison to confine him when he has misbehaved, or is it his special, secure place which he frequently uses on his own for naps or resting?
The doors to Indy's and Ava's crates are always open except when they are crated. The only time we crate them is when we are leaving the house, and can't bring them with us.
They are so in tune with our activities, that when they see us dressed up and leaving the bedroom, they automatically go in their crates. They are used to seeing me in jeans and t-shirts or other "ratty" clothes when I am around. If I put on slacks and a shirt to go out to dinner with Marcia, they will head for their crates.
Indy was a gem. It only took three days to crate train him. With Ava, we didn't have to do a thing. Indy did it all. We'd crate them together, and Ava never fussed. She just followed the lead of her big brother/cousin.
They each had a crate, but they generally shared the same one until she was about seven months old. The crates were adjacent. The first time we closed the door on the crate with just her in it, she showed no reaction. She was comfortable.
They can instinctively and intuitively from experience, when we are leaving the house.
When crate training, it is good to crate them at random times, and important to make it a positive experience. But once they are content and comfortable with their crates, I haven't found it necessary to crate them at times other than when they are left alone.
They have been crated for up to eight hours, and when we return, they patiently wait for us to open the doors to their crates. They may be standing and wiggling, but they never press against the door, or show the slightest urgency to get out.
There is one other circumstance when we crate them. If we are having a family gathering, we will crate them for a short time when people begin arriving, especially if there are small children.
They get excited when people arrive, and though they are generally well behaved, their exuberance can make some folks uncomfortable. Once they get used to seeing the "strangers" in the home, they are let out and they will greet people politely. It only takes ten to fifteen minutes in the crate.