Most movies which are based around historical events take a fair bit of artistic license, but generally speaking, it shouldn't detract from your enjoyment of the film (which was made for your entertainment, after all). For example, consider movies like
Pearl Harbor,
Saving Private Ryan or
Titanic which are "based on actual events" and inject a fair amount of extra characters and story which clearly never took place in real life, but are considered thematic in relation to the historical event.
Movies will never be exactly true to life. History isn't that exciting. I believe that provided that the filmmaker stays true to the theme of the actual event, a bit of theatrical exaggeration is not only inevitable, but desirable. Of course, he or she also has a responsibility to either say that content of the film is "based on an actual event" or if it is just a period piece that is entirely ficticious.
With regards to
The Patriot, I do not recall the movie ever claiming to be historically accurate beyond being a period piece. You can read some commentary on the film and history
here,
here and
here.
Gettysburg is a fairly historically accurate film, if a bit sanitised. Based on Michael Shaara's book
The Killer Angels, the film is fairly accurate in terms of its themes, characters and period depictions. In part, they got it so right because Ted Turner (the producer) is a big Civil War buff and he in turn enlisted the legions of re-enactors who have gone to great lengths to make sure that the costuming was just right.
The films greatest shortcoming is not its own fault: because the movie was made for television, the battlefield carnage is fairly clean. In reality, Civil War battlefields were horrific. The .58 minie-ball which was used by most of the rifles on both sides caused massive damage upon impact; if you were hit in the arm or leg and the bullet struck bone, you were almost certainly going to become an amputee. If a bullet struck someone in the torso, the entry wound was often about the size of a quarter, but the exit wound would probably be about the size of a fist. In addition, artillery fire, which was responsible for the vast majority of wounds and deaths didn't just throw people in the air as depicted on film, but tore them to shreds. This sort of gore, however realistic, simply cannot be portrayed on television, so this level of "cleanliness" is inevitable and probably would have detracted from the story.
Of course, some people were incredibly lucky when being shot. Joshua L. Chamberlain (portrayed by Jeff Daniels in the film) was wounded twice at the siege of Petersburg. The first time, his horse was shot thorugh the neck and the bullet struck a mirror in his breast pocket, grazed his arm and then struck another person. The second and more serious wound happened a few weeks later when a bullet hit Chamberlain in the hip, passed through his bladder and then went out at his other side. Chamberlain stayed on the field using his sword as support until he passed out. He was carried to a field hospital where a doctor declared that he would die from his wounds. LGEN Ulysses S. Grant gave then-COL Chamberlain a field promotion to BGEN and sent him off to die (or so everyone believed). Chamberlain ended up living through the night and was sent to Washington to recouperate. Chamberlain returned to duty and was eventually promoted to MGEN and was given command of the Union V Corps. He retired and served four terms as the governor of Maine, but the doctor was right, in 1914, Chamberlain died from complications related to his old war wounds.
My other major complaint about the movie (and the book, too) is its hagiographic portrayal of Robert E. Lee. This too isn't really the fault of director/screenwriter Robert Maxwell, nor is it really Shaara's fault. Lee is an iconic figure throughout the old Confederacy and after his death in 1870, he was elevated by writers and his peers to near-demigod status (something he would have hated) and historically has been above reproach. In historical circles, saying something bad about Lee is a political no-no just like saying something good about Hitler. The scene where the Confederate soldier are seen cheering around Lee before the third day is a litt bit too far over the top for me.
Personally, I don't like movies that try to portray actual events. Most often times they come up sort, just like movies based on books. Movies are primarily entertainment, and so they often take liberties with history. It's when they try to pass themselves off as history, that they start to bother me.
The Romans kept very good records. Should I compile a list of the historical inaccuracies?