Big dittos to JR and I upvote his answer. Let me just add:
Adding any additional phrases tends to weaken the statement, that is, make it sound less certain. Consider these phrases:
We should declare war on Trantor.
I think we should declare war on Trantor.
From my perspective, I think we should declare war on Trantor.
The first is very emphatic: this is how it is, period. The second makes it an opinion, and is thus weaker. The third is weaker still.
If your point is to make clear that you are uncertain or that you acknowledge contrary opinions, then add the qualifiers. But if you are trying to be persuasive or forceful, you should omit them.
Also, as JR says, your phrases are not interchangeable. "I reason that ..." indicates that you have made a logical analysis. You might say, "I reason that the president's proposed trade bill will reduce unemployment by at least 2% because ..." followed by some evidence. You probably wouldn't say, "I reason that strawberries taste better than cherries", because there is no analysis here, just a personal preference.
"Postulate" is normally used to indicate that you are proposing a theory for which you do not yet have strong evidence. Like you might say, "I postulate that there is life on the planet Aldebaran VII", meaning that you have reason to believe this is true but are far from having proof, or even that you are just throwing it out as an interesting topic for discussion with no evidence at all. You wouldn't say, "I postulate that ..." for something you firmly believe to be true. Like a politician would be unlikely to say, "I postulate that Mr Jones is the best candidate". He might say, "I am convinced that Mr Jones is the best candidate" or "I have concluded that Mr Jones is the best candidate."
Etc. Look at the definitions of each word before trying to use them.