No! You don’t! Serger looks fancy and it does add a nice finish to the seams, but it absolutely is not necessary to make beautiful knit garments. To illustrate my point I have made two almost identical Misha Vests. Well, almost identical. One vest is size 9 months and the other 12 months.
I used scrap sweatshirting fabric for both vests. The ribbing and the heart applique are made using the wrong side of the fabric. The smaller vest on the left, size 9 months, is made using a serger, and the larger vest on the right, size 12 months, is made using a zig-zag stitch on my sewing machine. I think it is safe to say that the vest made using zig-zag stitch looks just as good as the one made using serger! But let’s take a closer look.
Here is the photo of both vests from the inside. The vest made using a serger may look nicer to some, but barely does to me. The vest of the right, the one made using zig-zag stitch, looks just as nice to me. The look from the inside certainly does not take away from the functionality and stretchiness of the vest.
With a serger, I simply serged all seams. Whereas with a zig-zag stitch, I first sewed the seams together and then finished the seam allowances with a wider zig-zag stitch. So I guess there was an extra step involved, but it really did not affect the sewing time that much.
I do like how on the zig-zagged vest the thread is almost invisible as it blends into the fabric so well.
If you really do not like the way the zig-zag stitch looks, the only place it is visible is the back neckline seam. But still, none of it is visible when the garment is worn.
If you do not have a serger, or prefer to use zig-zag stitch for sewing with knits, I recommend setting your zig-zag stitch to 1.5 wide by 2.5 long for all joining seams. After you’ve sewn the seams you can finish the seam allowances together with a wider zig-zag – 2.5 wide by 3 long.