Neglecting to prune roses does little harm, with a few exceptions. One of the main consequences is the potential to have a garden full of overgrown rose bushes. Because you should only remove one third of a rose plant's height at a time, neglecting pruning for several years can lead to a bushy or leggy plant. This is not a problem if you want your roses to grow large, and once they've reached maturity, it's best to let them remain at that size.
Before planting roses, choose your variety according to your desired pruning schedule. Grandifloras and hybrid tea roses tend to need lots of pruning, because removing part of last year's growth -- namely, the old, vulnerable wood -- helps these roses bloom on stronger branches in the future. Meanwhile, modern shrub roses and groundcover roses prefer a little neglect. Though you can lightly trim them to control size if desired, the only time you must prune them is to remove problem branches.
Pruning your roses can help improve their overall strength. Stems are sturdier and blooms larger when you perform regular trims, either after the growing season or during the dormant period in winter, depending on rose variety. Trimming also allows you to shape your roses to suit your garden's style or size, while thinning branches -- such as removing dead canes -- can improve air flow and overall health. However, avoiding this type of pruning doesn't automatically mean your roses will be unhealthy. If you prefer a hands-off approach to gardening, you can allow your roses to grow to their natural size and bloom when they like, without trimming to make larger flowers form.
Occasionally pruning is a must, no matter what type of roses you have -- and neglecting these trims can spell doom for your plants. The main reason to prune immediately is to remove a diseased branch, before the disease can spread to the rest of the plant or garden. Pest problems and limbs that are crossed, broken or dead also require quick attention. When performing this type of corrective pruning, disinfect pruning tools between each cut so infection doesn't spread.