Ideally, roofs should be designed to direct snow accumulations away from entries and walkways. But valleys should be avoided wherever possible. And if you insist on valleys, you should plan on using extra-wide valley flashing, holding back your roofing at least a foot on either side to provide an easier path for the snow to shed. Dormers can present their own problems. Gable dormers, particularly in pairs, can create a choke point for snow to accumulate.
Shed dormers are better.
But snow dumping from a shed dormer to the roof below can cause damage, particularly if large chunks of ice have formed. If valleys or dormers are unavoidable, additional measures can be taken to reduce snow accumulations and potential for leaking. Most people think of insulation as a measure for saving energy. But roofs that are well insulated are also less prone to leaking. Heat escaping through a roof can cause snow to melt.