I have notice that many Catholics think that if something is not "de fide" teaching, that they are free to reject it!

This is a serious error.

For instance, the Baltimore Catechism teaches SO many truths, and there are no labels as to whether something is "de fide" or not, so how do we know which is and which is not?

We don't know....

We are not supposed to know which is or is not - because that is for confessors to use. We are only supposed to know that we are obliged under pain of mortal sin to believe everything in the Catechism because the pope approved of it for general Catholic learning.

Now, take a look at this approved chart of the various theological notes attached to teaching that was designed for confessors:
http://genus.cogia.net/Cartechini.pdf

Take care to notice that even though something may not be labeled with "de fide", it can otherwise be labeled as:
proxima fidei
theologicae certum
doctrina catholica

The chart shows that those who reject teachings labeled as such, would still commit a mortal sin, as would anyone promoting doubting what is taught by the Baltimore Catechism.

So, would you dare, for example, to reject the doctrine of guardian angels because you doubt it is "de fide"?

If you dare, it is a mortal sin, and you choose hell by doing so!

Think about how serious this subject is.

Are we free to deny what is not "de fide" teaching?

Strictly speaking, yes.  It depends on the topic.  In your example of the guardian angels, based on my limited knowledge, i'd say it's 'close to the faith' therefore we would be stupid to reject it.  But, must we accept it, under pain of sin?  It's not in the creed and it's not in the mass or the rosary, so i'd say 'We have to believe in angels because they're in scripture, but we don't have to believe in the idea of a personal guardian angel.'  Just my opinion.

You are a very angry person.

 In your example of the guardian angels, based on my limited knowledge, i'd say it's 'close to the faith' therefore we would be stupid to reject it.  But, must we accept it, under pain of sin?  It's not in the creed and it's not in the mass or the rosary, so i'd say 'We have to believe in angels because they're in scripture, but we don't have to believe in the idea of a personal guardian angel.'  Just my opinion.

Looks like Bumphrey is here to condemn rather than to inform.
However, the idea of a personal guardian angel is in Matthew 18:10:
"See that you despise not one of these little ones: for I say to you, that their angels in heaven always see the face of my Father who is in heaven."

However, the idea of a personal guardian angel is in Matthew 18:10:
"See that you despise not one of these little ones: for I say to you, that their angels in heaven always see the face of my Father who is in heaven."

Great point.  

So, putting aside my heretical response, which I reject wholeheartedly, (please, bumphrey, will you absolve me?), as to the original question, i'd say "it depends".

Sure.  I'd be happy to respond ... if you unban me from this thread.

This is the 3rd time you have violated your own professed principle. You can easily PM whomever you need to (or start a new thread) to communicate.

As well, you aren't banned. Banning is done with force, where a person has no choice but to be excluded. In your case, you professed that if someone asks another to not post in a thread he starts, that one should willingly comply. You have, so far (3 times) willingly violated your own professed principle.