I've seen a lot of questions lately around what needle size designers should put in their patterns. There are a lot of factors at play - do you consider yourself a tight or loose knitter, did your testers use needle sizes across the board, did you use a needle size much different from the size recommended on the yarn label? It's easy to get confused and doubt yourself.
Luckily, the answer is straightforward:
It's generally best to give the needle size that you (the designer) used. (Because you're also going to include the phrase "or size needed to obtain gauge", right?) That said, if you do tend to be a tight or loose knitter, you might want to note that so knitters can adjust their needle starting point accordingly.
As a designer, your goal is to present information as clearly and consistently as possible to your knitters, and you want that consistency between your patterns as well as within each of them. If a customer likes a pattern they purchased, and want to come back for another, they'll appreciate if your patterns are consistent. It's OK if they knit at a different tension than you. Over time they will still come to trust that when they knit your patterns, they will always need to go up one needle size (or whatever). Contrast this with choosing your needle size based on what testers say - you may have a different batch of testers from pattern to pattern, and the needle size may vary as a result. A knitter may find that for one pattern, they used the given needle size, but for another, they had to adjust, which can cause frustration and seems less professional from the customer's viewpoint.
What about using the needle size on the yarn label?
You can do that. Some designers do. I'd keep in mind that you may have some inconsistency between yarn companies. I definitely wouldn't take this approach if your knitted fabric was different than normal for a given yarn weight. Lacy shawls knitted with sock yarn require a very different needle size than is typically on a fingering-weight yarn's label.
What if I don't want to give a needle size, so people have to swatch?
You can do that, too. Consider if your target customer would be OK with that, or would feel more comfortable having a starting point. You do still have to include the type of needle in the materials list, such as circular (including length), DPNs, etc. This would be worded like "One set of 24-inch circular needles in the size needed to obtain gauge."