Depends on your genetics. Some can, some can't. We have a fair bit of hills in the PNW, but usually only up to about 7%. 10% is the exception, and only short bits of 15%-18%. Even so, I don't ride with anyone who uses a double. It's all about ego, IMO. The more you pay for a bike, the smaller the stock cassette. Ever notice that? And bike shops are big on ego. Get the big cassette. Heck, get a 32. You'll probably need a different RD, but save the original. After a year or two, if you find you never use the last couple of cogs, get a smaller cassette.