Why do you need to dig? We rarely touch our borders. We normally sprinkle slow acting fertiliser over them when the top growth has been cut down and then cover everywhere with a thick layer of organic material. That way nothing is disturbed and the weeds grow in the mulch so are easier to remove.

The only time I would want to dig my garden in the winter is when I have vacant, heavy soil, which I would roughly dig to allow the winter frost to break it down. I would wait until the spring to mulch and fertilise as most of the nutrients will be washed away by the winter rains.

Agree with the first two answers,but would add, organic mulch is usually placed over the soil while its got some warmth, if you do it in autumn, and the soil has already begun cooling now. In spring, it can be laid over the top of granular fertilizer, or laid without fertilizer beneath, when the soil is moist or damp as the growing season gets going. Fertilizing in autumn or winter isn't advisable - applied in autumn, fertilizer may encourage plants to put on growth that won't have time to harden off before winter sets in, so as a general rule, fertilizers are only used on the soil between March up to mid/end June, or not after the longest day. There are some exceptions - applications of sulphate of potash for instance on things like raspberries will be done out of the main growing season, but general fertilizing isn't done then.

If you're planning on stripping out the border, removing all plants, digging over and preparing for replanting, that can be done now or over winter, with the planting taking place around March/April, depending on weather conditions, but even then, you wouldn't apply fertilizer till spring.

If you need to dig established borders in order to remove pernicious weeds, that's best done in late spring, by which time you can see where the bulbs are and where the weeds are growing again, but otherwise, I'm a fan of the no dig method mentioned by Owdboggy above.

The old rule used to be 'Mulch in autumn to keep the warmth in and manure in spring to warm the soil up.'
I add slow acting stuff now because in Spring the mulch stops it getting to the plant roots quickly. In Spring I would use a quick acting fertiliser rather than a slow one if I was going to add any at that time.

Interesting,Owdboggy - to be honest, I often don't bother with granular fertilizer at all in spring, but it depends what the mulch is - if its something like bark chips or wood chippings, then I do put it underneath, but if its manure based, I don't always, unless its apparent its necessary.

A lot of our borders have been neglected over the last couple of years and so this winters task (weather permitting) is to 'tidy' them. It can't be done in summer because this would involve damaging too many plants.
The tidying will mostly involve a hand fork but in some cases the border fork is involved. The herbaceous plants should not get disturbed, any accidents are potted up and go into the greenhouse over winter. Some bulbs are inevitably disturbed but they just get shoved back into the soil and, usually, come away all right in the spring.
I don't 'dig' as in the veggie garden sence and would not even be doing this if I didn't have to!

Adding a good layer of mulch at this time is a good idea, especially if you get a dry snowless winter. A couple inches of mulch now will buffer your plants against a hard freeze, help retain moisture, warmth and prevent heaving - all good for plants. Even though the plants go dormant, they still need moisture to survive. For fertilizer & digging, wait until Spring or follow Bulbaholic's advice.