Do you want this fence covered all year round? If so you'd be better off choosing an evergreen shrub such as junipers, aborvitae, etc. You have many options there. Grasses will need to be cut almost to ground level and will take a few months to get going again. Vines would also be nice. If it's not full coverage you're looking for I'd go with a Miscanthus cultivar.

Jen

If you prefer a more vertical screen along the fence line you might like planting a row of ..
Calamagrostis x acutiflora 'Karl Foerster' - Feather Reed Grass.
The seed heads remain attractive throughout the seasons including winter. Of course you will have to give it time to reach the height you require but it's vertical growth habit would allow you to also plant other things in front of it if you desire.
Pampas grass will easily hide a 6 foot fence but also covers a large circumference at the base. It has it's virtues but is also considered an invasive species in many areas.
Let us know what you finally decide on planting.

I agree with both. You'll never get year round coverage with grasses so if you can add some evergreens I would. Many go great with grasses as long as you don't plant large grasses too close to shade out the conifers. The winter contrast is great also. Even if you don't think you have the width for evergreens there are tons of narrower ones. Adrian Bloom uses a lot of both and his gardens look awesome in books.

But if you just want grasses, do a mix of cool season growers like Karl Foerster and warm season growers like miscanthus. The cool season growers put on fast growth in the spring and early summer and then the warm season growers kick in. You can put larger miscanthus in the back with the Karl Foersters in front then a lower grass like fountain grass in front of that. Mix it up and have fun. Some grow pretty fast so you don't need to buy big to get a quick impact in 1-2 years (many miscanthus cultivars and pennisetum come to mind). Get some books also.

Bill