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.
When pruned well - and sparingly - these shrubby Veronicas can enhance any garden - or container. There are easy maintenance, generally trouble free flowering evergreens that just need a bit of sprucing up occasionally. This is what you do - and when.