Kakashi tells Jiraiya that Rasengan is a dangerous technique after he stopped Naruto and Sasuke who fought for the first time. If it is that dangerous, then why was Naruto allowed to teach Rasengan to Konohamaru?
Although Rasengan may be a very dangerous technique, it's not a forbidden technique, so anybody can teach it to everybody.
The Naruto wiki has the categories for forbidden Jutsus:
Techniques that cause harm to the user themselves, such as opening the Eight Gates, which the mere use of is both highly useful as well as detrimental to the user.
Techniques that violate the laws of nature (e.g. the Summoning: Impure World Reincarnation, which reincarnates the dead with a human sacrifice).
Certain techniques that are known to cause massive collateral damage, such as the total destruction of a village and end with the death of everyone in it, thus the great moral ramifications of its potential lead many to labelling it as a forbidden technique.
Rasengan does not fit in any of these categories, so it's most likely not a forbidden Jutsu ;).