Sweater seams

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

I like to slip the first stitch of every row I work, no matter the pattern, this gives a flatter neater edge. I never add any additional stitches, though, unless slipping the first stitch would somehow mess up a pattern (say for colorwork or for complex lace). I don’t think your friend described the process correctly to you though, I don’t think you’d have 2 additional stitches at the end if you slipped the first stitch and then worked the pattern, I think you just have one additional stitch on each end.