Would Americans in the 21st century react differently to the issue of "no taxation without representation" than the colonists did?
The American reaction to a similar situation today would be similar to that of the colonists. If anything, modern Americans might react even worse than the colonists did.
In colonial times, the American colonists were not represented in Parliament. They had their own colonial assemblies, but they did not have anyone directly representing them in the Parliament that met in London. Therefore, when the Parliament started imposing taxes directly on the colonists, they felt that they were being mistreated. They reacted in such ways as abusing the tax collectors and refusing to buy goods that came from England. Eventually, of course, this issue helped lead to the Revolutionary War.
Today, Americans are strongly suspicious of taxation. In addition, they tend to distrust the government. They do have a relatively high level of trust in the people that they elect themselves, but they believe that the government as a whole does not have their best interests at heart. Therefore, if taxes were imposed on them by a government made up only of people they did not elect, Americans would surely become very angry and rebellious.