Ideally, yes. For two reasons:
1.) I don’t think it is reasonable to expect all the features of a dedicated text editor in an app such as Blocs. Editing text in the dedicated editor of your choice will provide many features such as seeing non-printing characters, applying case changes, expanding abbreviations for frequently used snippets, search and replace, key shortcuts, markup tools, comparison tools, and much more.
2.) As with any text designed for the web, style runs that may be applied to text on the scrap, as well as non-printing chars embedded in some text encodings might create havoc when pasted into a web layout tool. The best way to avoid this is to paste text from other sources into a text editor before inputting it to Blocs. This is especially important when a client emails you copy.
I compose the text of my sites solely in a text editor. It is simply easier and faster, especially for large sites. It also makes it easier to update text as I can do a search and replace if I need to change all occurrences of “KMS” to “KMS, Inc.”, and in an instant know exactly what pages in blocs to paste the changes into. Like the graphics used, these are source documents for the site. We don’t edit graphics in Blocs, and I have the same mindset for text. It is simply a matter of choosing the best tool for the job at hand. A good text editor is just so much more responsive and intuitive.