There’s quite a bit of misinformation about whether using dashes instead of underscores helps as part of your larger SEO strategy in giving you higher rankings on key words or phrases. Some articles say it does, others say that underscores are better. Some put a lot of weight on this point, whilst others don’t (although it should be noted having search-engine friendly URLs, or Permalinks in WordPress, are a must).
I came across this excellent video from Matt Cutts, who works for the Search Quality group for Google (if you haven’t heard of Matt, do a Google search – he knows what he’s talking about here).
Take a watch of the video below; it gives a pretty clear answer as to what you should do if you’re:
To summarise Matt’s video:
Furthermore, Matt recommends using dashes as seperators in a URL – but even if you use underscores and you’re getting the traffic and results you require, leave this as it is.
The key, critical answer to this question?
It’s not a primary factor, and there are over 200 other signals Google uses to determine the search order for any given term or phrase (you might need to watch the video a second time – this is such an important point but he only briefly mentions it).
For example, if your page has a low PageRank, your code isn’t search engine friendly or perhaps your content doesn’t have the keywords you want to rank on, the URL structure is an unimportant point.
If in doubt, use dashes in your URLs.