The overall American grievances that led to the Declaration of Independence were valid, but many of the actual claims made against King George III in the Declaration itself were probably overstated.
The basic American grievance was a desire for more self-government and autonomy. This was certainly a desire that was warranted. The colonies were clearly able to govern themselves. Their people had the education needed to rule themselves and they had ample cultural background (being largely English) in democracy. Therefore, they were certainly "ready" to rule themselves democratically.
As for the specific claims made against the king, we must keep in mind that this was a political document. Political documents do not keep strictly to the truth and this was no different. There is validity in the grievances, but they also go farther than is warranted.
I think one might successfully argue either side of this question. Certainly, there were overstated grievances, and the document excuses any bad behavior on the part of the colonists. However, I believe the trump card was the idea that government gets its power by consent of the governed. Indeed, it may be argued that had the colonies been granted a place in Parliament, independence groups like the Sons of Liberty may have not garnered the support they gained during the period leading up to the revolution.