Typically we are taught to do a chain 3 as the first stitch for a double crochet. The chain is what creates the visible seam, in addition to how it is joined to the beginning stitch. The circle to the right on both images, are created with the chain 3 which counts as the first double crochet. I have found that I like to do a chain 1, which will not count as a stitch, and then do a full double crochet. I will use this same technique with the half-double crochet to. I have found that the chain leaves larger gaps and holes that contribute to a visible seam. By doing the chain 1 and a full stitch, this technique will fill in the gaps that a chain will leave. When you complete your round, you skip over the chain 1 and join your round together in the top of the first stitch. In my examples, this will be the double crochet.
With the image on the right, the boxes highlight the starting stitches. If you click on the image close up, you will see the chain facing you. As you progress in your project the seam becomes more visible with each round. With the chain 1, double crochet, you have a full stitch with a short chain that is less noticeable.
Click on Image for Close View
How we join also plays a big role with a visible seam. The image below is showing how to join to get a less visible seam. This can even be done with the chain, if that is the technique you are most comfortable with. It will twist the chain to the side, instead of being to the front, as shown with the circle, on the above right. You put the hook under both loops at the top of the chain or stitch, wrap the yarn over the hook, pull it through all loops.
No matter what method you choose, there may still be a slight visible seam with the basic single crochet, half-double crochet and double crochet when you are working with joined rounds.