I used to panic on this. The best commonsense advice I got was if the roses are looking a little like they can't compete then pull up the flowers that are enroaching - not a problem. Competition might come in terms of: roots, airflow and sunlight.

Obviously nothing deeprooted or a chore to pull up like agapanthus is a good idea. I check out the estimated height of the rose bush and use it as a rough guide to the expected diameter of the plant for spacing out the rose bushes and the plants that I consider more effort to pull up. Also consider not planting annuals or bulbs close to roses that will involve disturbing the roses roots all of the time and other similarly over vigorous plants that will create problems for light and airflow. I think the whole cottagey gardenbed thing is just go rampant: things will seed and grow well where they are thriving. Once plants start to crowd each other a little in the beds I trim things up, snip up the cuttings and add them to the top layer of mulch. I don't know of any companion plantings that might create problems for the roses by leaching out nutrients and considering the fertilizing roses get probably not an issue. Low ground covers like allyssum can be planted right to the base of the rose, think of them as living mulch. I don't really see any problems with planting less vigorous, below 30cm flowering plants close to thriving roses either the rose will tolerate them or not and it might make them a little leggy. Short ones should be fine, trees and bushes that will grow larger than the rose present problems. When choosing companion plants obviously they will need the same or similar light, soil and water requirements. Plants susceptible to fungi I wouldn't recommend as a nearby plant. I'm mystified by rose growers who pair roses with climbing clematis, providing it isn't too vigorous it works.

In the real world of nature plants don't have a four foot square drawn around them with a keep out sign and nor do plants die should a blade of grass have the temerity to enter its growing space. Guaranteed some roses are hardier than others. I think just pay attention and give the rose priority to other plantings at need and experiment a little. I'll let you know if my petunias assasinate any roses, but I'll bet they loose the war. The current rules are anything that is within a footstep of the roses foliage goes - but its more of a room for the expanding bush thingy. I suspect the roses thrive more when given the space tho and you want to be able to get in there and snip when you wish.