Rosecrantz and Guildenstern come into the play seemingly from nowhere. While they claim to come just for a visit with their old friend from childhood, the audience knows--and Hamlet discerns rather quickly-- that they are there at the king's request to figure out why the sweet prince of Denmark acts so mysteriously.
They certainly aren't heroes, for they have nothing of nobility about them. They engage in jesting, seem more interested in the pleasures of life than the important matters of State, and, most importantly, they are willing to sell out a friend in exchange for favor from the king (whom we know is beyond corrupt.)
In a sure sign of Hamlet's sanity, he reverses their fortunes, and they become the victims of the note written by Claudius--intended to result in Hamlet's death.
Could they be symbolic? I doubt it. It would be stretch to call them symbols of an uncertain time, but they could be considered symbolic elements of foreshadowing because King Claudius' attempts to kill Hamlet turn against both his will and the deliverer of his wishes. Here, Rosencrantz and Guildenstern willingly take a letter to England that is supposed to result in Hanlet's death. Hamlet turns in around, and they die by that very decree. In the final Act, Claudius and Laertes conspire to poison Hamlet by one of two ways (sword and drink,) but again, that plan ends up killing both Laertes and Claudius (along with Gertrude and Hamlet.)
I doubt Shakespeare had such symbolic intentions, however. I suspect the duo served a means to an end--get Hamlet to England, reveal the character of the evil King, and add a little levity to the tragedy.