It depends. If you know positively that's the profile that will be used, you can do that (a lot of people use it just because it happens to be the Photoshop default).
Pro: you'll be working within Web Coated SWOP's limited gamut from the start, so you won't be disappointed when some colors are clipped in the final conversion. And they would be, because US Web Coated SWOP v2 is a very small color space. Don't expect deep saturated blues, for instance, because you won't get it.
Contra: You have to be very careful to not exceed TAC, total area coverage, while you work. This is the total amount of ink the paper can hold without smearing and/or drying issues. If you convert from an RGB profile this is taken care of automatically, because TAC is specified in the profile, but an adjustment in CMYK can easily push you over the limit. Can't recall what the specific limit is in this case, but somewhere around 300 to 310% should be safe. Ask the printer. You have to monitor this continually.