Mulching roses is quite important for a number of reasons. Notwithstanding the fact that 'mulching' is generally a good gardening practice and shows good plant husbandry!
The mulch used, can be any form of rotted organic material or even a used potting compost. In days gone by, horse manure was considered the best.
Mulching to a depth of several inches helps to keep the root system cool and moist - particularly in thin, dry soils. This has a beneficial effect in combating Rose Mildew.
Mulching roses has the beneficial effect of keeping the lower part of the root stock moist, which encourages basal growth, thereby constantly regenerating the rose bush from new ground level shoots.
All mulch materials will eventually rot down into the soil and be transformed into food which can be taken up by the rose. This is of great importance, for regular pruning of roses removes large reserves of plant nutrient from the soil as the bush has to constantly renew itself.
Mulching materials should generally be well rotted. Un-rotted straw for instance - or worse still, sawdust - will take nutrients out of the soil during the process of rotting and decay. Whilst these types of mulches will 'eventually' benefit the rose bush, the interim damage done as a loss of soil nutrient can be quite severe. If such mulches have to be used, then ensure that supplementary feeding is also carried out to counteract the problem of soil nutrient loss during the mulch decaying process.
Grass clippings are sometimes used. However, as they often take months to properly decay, they can turn into a mini thatched roof over the area, thereby shedding the rain water away from the roses root system! Not recommended unless properly rotted down first.