I run a blog on blogger.com. It gives me options to include tags in each post I make. Would it be a bad idea to have the same tag for each post or should I change them?
If your posts are all about the same subject, then they could well have the same tag. For example, all of your posts about 'widgets' could all have the tag 'widgets'. If you had other posts about 'poetry', then that would be a better tag to use for those posts. You might be able to use multiple tags for posts too, e.g. 'blue, widgets' or 'funny, poetry'. Does that make sense now? Cryo.
i use wordpress and it has enough plugins to automate or simplify this tagging process blogger.com is too controlling
Using tags is always a good idea. I always tag my posts with the main subject, and then anything a few more relevant tags I can think of. Like someone said above, if your blog is about widgets, it makes sense to have every post tagged as "widget". This helps people find your posts with tools like technorati.