If you need to share electronic versions of your documents with others, you need to consider what they are using. If they are predominantly Word users, then using doc or docx will make the sharing easier. Another factor to consider is how complicated your documents are and what features you are using. If they are complicated or are using a lot of less common features, the conversion to a Word format may lose some formatting or features.
You can set LO to default to the doc, or docx, or odf format, so that you don’t have to select it each time.
An option if you only share some documents sometimes, is to use odf as the default and save as doc or docx only when necessary. Alternatively, if the others do not need to be able to edit, you can use odf as your default and export as pdf to share.
As to @rautamiekka 's comment: I agree. I use MS Office 2003 which saves natively in doc, but can support docx through an MS supplied add on.
[EDIT]: Also see this post: Docx issues