Most Hebes need little or no pruning. However the larger-leaved types are prone to damaged stems, and these can be safely cut back at any time during the growing season.
Dead-heading of old flowers on Hebe plants is well worth the effort. It can extend the flowering period, or encourage a further show of flowers later in the year. If your particular plant becomes straggly, then cutting back hard - to within 30cm of the ground - will rejuvenate it. However, this last option should be done with care! See below.
As with the vast majority of flowering evergreen shrubs, Hebe shrubs should be pruned in the growing season - or right at the end of the winter dormancy period. They should not be pruned in late Autumn - nor winter months.
Hebe eliptica. This and other similar Hebe bushes, often suffer from snow damage, which opens up the centre of the shrub and often causes some of the branches to snap off under the weight.
Normally it is enough to simply cut out the damaged stems, and regardless of the shape after doing so, it will soon regain the compact habit of growth for which they are known. After flowering - which can be quite prolific, the fade trusses should be snipped off. This will allow flush of new growth - and also prevent the untidiness of this particular variety going to seed. Allowing the seed heads to develop will also take away some of the vigour from the shrub.
Hebe Midsummer Beauty and other similar long leaved Hebe varieties, generally require no pruning other than cutting out winter damage. However, the shrubs tend to get a bit top heavy, with bare stems at the bottom of the bush.
Cutting back a few of the older stems to low down - near the base, will allow new branches to form, which will better clothe the shrub from top to bottom. Dead heading of the flowers as soon as they fade, will ensure continuation of the flowering period.
Hebe Tricolor is one of the less hardy types of Hebe shrubs that will probably have to be cut back quite hard after a hard winter. There will normally be new shoots emerging from lower down the stems, so pruning back any dead wood should stop at the new growth buds. These will soon break out to form new branches.
However, larger growing varieties can suffer damage in winter months - either by weight of snow or severe frost damage. In this case, if it is not sufficient to prune out the damages stems/branches, they can be cut back quite hard. This is best done in two stages. Firstly cut around one third of the branches quite hard, then when the these start to re-generate into new growth - usually after just a few weeks - the remaining two thirds of the branches can be pruned out.