Can I?
Should I?
When?
Yes you can, light pruning/deadheading can be done pretty much anytime of the year.
If you are looking to do a harder prune back into old wood it is best done when the plant is dormant, say Jan/Feb. Regrowth can be a bit hit and miss and you may sacrifice a years flowers.
If the plant is really out of shape you can cut it back to ablut 30cm from the base and it will re grow though some suggest total removal is best and replant a new specimen.
I have sucessfuly pruned a very large small leafed hebe, lost of tiny white flowers and only about 40cm tall but 2m across. I pruned in Jan back into old wood and it took a year before it had totally greened up again.
HTH
Jerry
Pruning (other than mere tidying up) of Hebe is best done in spring when the sap is just starting to rise. Try not to prune back beyond leaf axils with live leaves - if you prune beyond that the shoot is likely to die back. We have one Hebe on which we have used this technique successfully at 5 yearly intervals for over 35 years.