There is great debate on whether or not cultivars of native plants are acceptable to use in a native landscape. The term which is coming into use is 'nativar'. A nativar is a plant which is a close cultivar of a native plant.
Whenever possible seed-grown native plants, cultivated from local populations persisting in similar conditions, are the best choice. It is not easy to find seeds from your eco-region and when you do, how can you be sure they are pesticide free?
Here at Watermark Woods we typically look for the straight native species of a plant. There are times when we will stock cultivars.
Conditions may make a nativar as well suited to the application as the straight species, or possibly better. For example: Virginia sweet spire 'Itea virginica'. The nativar 'Henry's Garnet' is a great replacement if you have limited space. The blooms and leaves are basically the same as the original, but the stature of the plant lends itself to smaller applications. On the flip side of the coin, look at all the varieties of Echinacea which exist today. The colors of the flower alone are so numerous that insects that need the Echinacea purpurea would be hard pressed to find one that has not been genetically tampered with. There has been limited research to support nativars and their impact on insects, but if the traits insects search out such as, bloom shape, bloom color, leaf shape and leaf texture have remained unchanged then the insects should be able to find it.
In short, it is worth the effort to learn about the specific varieties you are planting and the differences between the true native and the nativar. Does it still provide benefit to wildlife or has it been altered to a point beyond recognition from the original?