A little trimming for practical reasons, such as feet and ears, is sensible. What you do NOT need to do is have your Cavalier clipped all over in hot weather - having long hair in hot weather makes no difference to your dog's temperature. On the contrary, hair trapped in the coat can help to keep your dog cool, just as it helps to keep him warm in the winter, by acting as insulation. Many people clip their Cavalier in summer because otherwise he puffs and pants when walking and therefore must be feeling the heat - then you look at their badly overweight dog and know exactly why he's puffing and panting! Nothing to do with his coat, everything to do with his weight. Apart from lightly trimming Aled's ears and feet, I do also trim his flanks because, being neutered, his coat gets very long, cottony and whispy there, so I just cut the long bits back level with the rest. Neither of mine have ever had a problem with messy feathers, but then they do both seem to have cast iron digestions and firm poos! Most of the light trimming you can do yourself with trimming scissors - we've had instances on this forum of groomers - asked for a light tidy up - who have got carried away and totally stripped the poor Cavalier! My two have never been near a groomer, they don't need it for light trimming and get bathed in the shower.
Kate, Oliver and Aled