The Rebellion cannot be truly good, and the Empire cannot be truly evil. They are concepts. They are political entities which act towards each other, and in response to each other.
Some might say the Empire is evil because of how it came about - it deposed a thousand generations of tradition and history, replacing it with a government that began as a constitutional monarchy, and migrated towards a pure monarchy. The Empire also was reportedly biased against non-humans.
The Empire isn't evil, because the Empire is a collection of people. Some of those people were evil, true. But I'm sure the same could be said for the Rebellion. In any guerrilla force, engaged in asymmetric warfare, you will find anarchists who simply want to burn anything tied to the 'establishment' and psychopaths who simply want to kill.
Since the Rebellion is best defined as "those who oppose the Empire and seek the restoration of the Old Republic" they don't get an automatic 'good pass', as the Empire isn't evil. Instead, we must consider their actions.
By and large, the Rebellion restricts its offensive strikes to military targets. They do not seem to engage in widespread terror attacks or in attacks against the civilian infrastructure. In this way, they are good. They choose to limit their effective choices for reasons both practical and moral.
That said, their goals may SEEM laudable, but are they really? Let's look at the Old Republic, and its guardians, the Jedi. The Old Republic had no standing military. The Old Republic was governed by a massive Senate. The Old Republic had no method of extending its reach.
The Old Republic outlawed slavery, ensured equal opportunities for non-humans, and provided for personal liberties. The Empire was known to use slavery in some instances (notably with POWs). The Empire was discriminatory against non-humans (though evidence for this is sparse in the movies, it's made much more clear in the Expanded Universe). The Empire was totalitarian, squashing personal liberties in some cases.
It would be easy to look at this and declare the Old Republic superior to the Empire...but I don't think it's that clear-cut.
The Old Republic was not a good government. The Senate was overly large, fractured into many hostile factions, and strongly influenced by corrupt corporations. When a veritable GENOCIDE was being committed by one of the Republic's worlds against another, the Senate did NOTHING. When a group attempted to leave the Republic, they were opposed with military force. At the drop of a hat, the Republic overturned most of its oldest laws to deal with a short-term threat.
Palpatine manipulated the system, twisted some events, and played people like a fiddle to gain power, true. That said, he COULD NOT have done so if the system hadn't been open to manipulation, had people willing to support his causes (NONE of the Seperatists were in on the whole plan, every planet which attempted to secede did so because of legitimate grievances or promises of a better situation for their people if they succeeded).
The Jedi were even worse. Consistently in the prequel novels and the movies, the Jedi act as Judge, Jury, and Executioner. They are shown to consistently kidnap children from their families, to raise them as Jedi. These children are brainwashed thoroughly, denied their families, taught to ignore their emotions, and taught to kill.
Given what they sought to restore, then, I don't think you can characterize the Rebellion as 'good'. In fact, in the Expanded Universe (which is now Legends canon, not the new Disney canon), the New Republic which the Rebellion formed after Endor lasts less than twenty (war-filled, economically shattering) years before collapsing.
At least, under the Empire, slavery was by and large eliminated, a united galactic economy emerged, piracy was brought to heel, and the space trains ran on time.