How/when to trim bushes that will eventually become a beech hedge?

For just over a year I've had a new beech hedge planted - the plants on one side were about 5 to 6 feet high when planted and are now about 7 feet - I'm aiming for a 6 feet tall hedge so do I just trim the tops by a few inches and leave them, or do I also need to trim the tips from the branches?

Other plants on the other side were about 3 to 4 feet tall so they don't yet need a height trim, but do I trim the tips (a inch or two?) off some of the branches?

 

Most people leave them until they're at the height/width they want them to stay at, but that also depends on how densely they're planted. Planted really close together, for example, they won't need pruned to get them bushing out.
I'd have cut those bigger ones down a good bit at the start so that they can spread, and bush out more, but it also depends on the look you want, and the aforementioned planting density. I never buy/plant hedging of that height though - always around 3 feet [1 metre] or so as they establish better. I wouldn't bother trimming the sides of any of them unless there's stems/branches getting far longer than is ideal for the space.
I've trimmed a couple of wayward bits on my new Hornbeam hedge, planted in late spring, but other than that, it'll be left until it's a lot bigger. It's still establishing, and won't do much until next year. It's very similar to Beech in growth rate and habit, but copes much better with wet ground than Beech.

 

Thanks, that's very useful.

So should I be cutting their topmost growing tips off to the desired height now?

The plants are about 18 inches apart.

Incidentally, some of the plants that I ordered were supposed to be about 4 feet, but what I received were about 5 to 6 feet, hence the height.

 

I expect you could trim the tops a good bit. As I said, it's not something I've ever needed to do, because I don't plant anything that size. It would let them put their energy into filling out, rather than getting taller, but personally, if I had plants that size, I'd cut them back further, although I'd have definitely have done it when planting.
It's your hedge though, and I don't know what it looks like, or know what your conditions are like in the site they're in.
Someone else might have a better idea about it though. It has to be your choice as to how to proceed.

 

Thanks for the advice.