Sweater seams

[franniebean]

An experienced knitter told me she always casts on 2 extra stitches when making a sweater that has a separate front and back (as opposed to being knit in the round). She then knits as follows:
Slip, K, (pattern), K,K
following this all the way up the body.
Is this common? Isn't the seam lumpy when you sew the 2 pieces together? She seemed to be saying that it makes the seams easier.
Would you do the same for arm holes, etc?
Thanks for any info