Best time to cut it back hard is spring, as the growing season begins. My book doesn't say you can't cut it any other time, but only talks about trimming it a bit. The trouble with cutting it during late autumn/winter is, if the weather turns mild, new growth may be forced by the hard pruning, which then gets frosted and burnt when the weather turns cold again. This applies to most plants, although usually its okay to cut back deciduous ones when they're dormant in midwinter.
I'll be interested to see if anyone knows another reason why your books say that.

The only other reason I can think of is to preserve the last of the flower, and the berry for the birds. Other than that, as BB says, follow the pruning evergreens route. However, being native, and well used to the seasonal vagaries, it won't sprout into new growth too early. Ivy's tough enough to take almost anything you throw at it, and if it's in your way, well, chop it. I happen to like the stuff, especially when it gets to flowering stage, but I grant it's not everyone's cup of tea. Worthy

I'd go with the flowering/fruiting response too. But as Bamboo says cutting now may cause tissue death too.
unless it is causing damage I'd leave it until the late spring.

I also go with Worthy, you will find it's a very safe place for small birds to live and nest, with many sources of food other than berries and flowers.

I join in with Db that ivy is a shelter for small birds in the winter, so is pyracantha too, but they have to be a bit bushy to perform this function. So puleeease leave them alone until threat of cold weather gone. Ivy berries are a favourite with the birds. Pyracantha possibly eaten later on so very valuable to birds.

i guess you could trim it as its quite a tough plant realy . id normaly weight till spring though as the ivy will have shut down and stopped growing . i guess if it realy realy needs it . trim it back a little till the spring or weight if you can .

I hack mine all year round and it grows even stronger, I use it for flower arrangements etc.

So do I, Grandmage. I never really thought it would harm it...tough as old boots! Subconciously, I probably did hope it would kill it! I really am 'going off' it now.

Izzy, the birds like to nest in it, so let them!!!!

a lot of people have trouble getting rid of ivy . i like it because of the nature but it grows naturaly here and would be hard to get rid of if i didnt like it i think.