I also often work the first 2 decreases only, 2 rows apart and then complete the remaining decreases according to the intervals given in the pattern e.g. decrease every 4th or 6th row. The reason for working the first 2 decreases close together is that this will give you a slightly wider shape at the v-point of the neckline. You can find sometimes that the neckband doesn't fit into the space at the v-point very well.
There are a number of ways to finish a v-neckline but one of the most common neckbands is a knit 1, purl 1 ribbed band. This type of band required decreases to be worked at the v-point. When I work this type of neckband I prefer to use a centre double decrease to give a neat mitred finish at the v-point.
In my post on picking up stitches around a round neckline I talked a bit about the rate of pick up i.e. how many stitches to how many rows to get a good finish. Picking up along a v-neckline you are picking up along a diagonal line which is longer than the same number of rows vertically. If I pick up 3 stitches for 4 rows which I would normally do for a straight vertical edge the neckband may look okay but the v-point will probably be pulled up. As the diagonal line is longer it is necessary to pick up more stitches so I usually work on a ratio of 5 stitches to 6 rows. You can pick up 1 stitch for every row but you may find this is too many stitches.
Picking up stitches on a v-neckline I would start at the top of the left front neckline/shoulder and with the Right Side facing me pick up 5 stitches for every 6 rows down the neck edge. When possible I put my centre stitch onto a stitch holder at the start of the neck shaping so I would knit this stitch but keep the stitch marker in the stitch as I need to know where this stitch is when working the neckband. I also take a note of the number of stitches I picked up down this edge. After knitting the centre stitch I pick up the same number of stitches (at the same ratio) up the right hand side of the neckline.
Once you have picked up all the stitches you have a choice. You can knit 1 row to give a garter stitch edge to your neckband which will sit back over your pick up row, or you can go straight into the knit 1, purl 1 rib. If you are going to work in rib it is important to get the rib correctly centred. The marked stitch at the v-point of the neckline must be a knit stitch on the Right Side of the fabric. With the Wrong Side facing you (which will be the first row), find the marked stitch which will be a purl on the Wrong Side. Having found the stitch you can now work back to the beginning of the row counting p1, k1, to work out what stitch you need to start knitting the band correctly.
Work one row of rib over all the stitches (making sure the marked stitch is purled on this Wrong Side row). I work the mitred decrease on Right Side rows only. The next row is a Right Side row. Work in rib to 1 stitch before the marked stitch as shown in the image below.
The centre double decrease is a knit decrease (even if the next stitch is a purl stitch) and is worked over the next 3 stitches as follows,
Slip the next 2 stitches together to the right needle (as if you were going to work k2tog)
Knit the next stitch.
Using the point of the left needle pass the 2 slipped stitches over together. NB they must be passed over the knit stitch together NOT one at a time.
By passing the 2 stitches over together you keep the centre stitch at the front of the decrease giving a vertical line of knit stitches with the decreased stitch either side disappearing behind this stitch.
After working the decrease continue in the rib pattern already set. On Wrong Side rows you MUST purl the marked stitch. On some Wrong Side rows you will have 3 purl stitches together but on alternate Wrong Side rows you will have knit 1, purl 1, knit 1.
An average neckband is about 8 rows. I usually cast off in rib on a Right Side row so that when I get to 1 stitch before the marked stitch, I work the centre double decrease and then cast off the previous stitch and continue with the rest of the cast off. This helps to keep the v-point neat and tight.
I hope that helps any of you who find it difficult to achieve a neat finish on your v-necklines.