My perennials are lucky if they get any sort of mulch and they usually make it through the winter. As a general rule, if they have anything alive and green above the soil I wouldn't dump a load of anything wet and soggy onto it. They might tolerate a mulch of dried stems, however.

Otherwise, it probably doesn't matter much. Especially if you are canny enough to have cuttings of all your favourite half-hardies snug in the greenhouse. And in the case of properly hardy perennials I don't think it matters at all.

So my general advice would be - chill. Don't make gardening harder work than it needs to be!

Well, actually:
(1) I don't have a greenhouse - so no cuttings - small cold frame already full;
(2) I lost some perennials last winter - lobelia and monarda - so clearly some do need protection;
(3) I have been doing autumn plantings in order to try to get plants better established in case of summer drought - so I have to bring young, small and therefore vulnerable plants through the winter. We have clay so winter cold/wet can be a problem.
Of course it is possible that we won't have any winter this year, that the four of five days of freezing fog we had before Christmas will turn out to have been the coldest days of the winter. But I really don't want to take the risk.

Points taken. But I don't know that mulching would make all that much difference in the case of monarda - it dies easy whatever you do to it, I don't think cold protection is the key.
I just wouldn't plant young, vulnerable plants in the Autumn.I might do it at the end of August/early Sept when we've had a bit of rain and there's plenty of warm weather yet to come. Or wait till late March/early April. If I had plants waiting and there was doubt about their tenderness I would pot them on in Autumn and keep them under a layer of bubble wrap next to the house wall so I could keep an eye on them. If you have clay, then waterlogging or slugs might kill things just as easy as cold.

Which Lobelia? The red L. cardinalis, including the red-leaved Queen Victoria, are extremely hardy. If that died, it wasn't from the cold. The blue (and pink and purple and white) perennial Lobelia hybrids, L. x speciosa etc, are not really frost hardy. Maybe they survive for you in winter but I'd give them some mulch.

Not sure how you could kill Monarda. Got any tips for me? :)

I mulch with compost over every patch of soil that I can get to. Soil is well drained, rotting is never likely to be an issue even with sensitive plants. Winter freeze kill isn't usually an issue either, I'm just feeding the soil at the only time of year I can see it!

You don't have to kill Monarda, it is quite happy to pine and dwindle of its own accord until it eventually expires in a haze of mildew. Or maybe I should take it personally.

It was the Hadspen selection of Lobelia, developed by Nori Pope at Hadspen in Somerset and bought at the garden. Presumably it is still produced elsewhere, now that the Popes have left and the Hadspen garden has been allowed to run wild. I do not know the species, I'm afraid, but a quick search on the net suggests that it may be Lobelia syphilitica. I believe that my mistake was to plant it (in August 2005) in a border that stayed very dry over the winter because of the drought. I had not realised at the time that Lobelias need constant moisture. I suspect I did not mulch it. My one surviving plant has been moved to another border that is always moist and is protected in winter by the house. There is some fresh growth now so I am wary of mulching on top of it.

Monarda I had no experience of before I planted it, and clearly now I am never going to have any experience of it at all.

We have clay, so I expect that I am always going to lose some of my autumn plantings. Maybe not such a bad thing when one puts in too many perennials in the first case.

mulch with well rotted manure or spent mushroom compost if you have no ericaceous plants, over time your london clay soil structure will improve. that's the main reason i mulch in london, hardiness doesnt really concern me at all.