This is a good question!
The taxes are present in the story because the tax collector comes to Miss Emily's home to collect her overdue taxes. She insists she does not owe any because Colonel Sartoris did not make her family pay taxes. Miss Emily has been so out-of-touch with the town and its people that she does not know that Colonel Sartoris has been dead for around 10 years. The men are rather intimidated by Miss Emily, as she is a stern woman who is temperamental. She is dressed in black and is intimidating. The men do not bother to tell her that Colonel Sartoris is dead and they do not insist any longer that she pay taxes. They leave when she instructs Tobe to show them out.
As for symbolism, perhaps the taxes could represent the town and the townspeople. Miss Emily has shunned the town and the townspeople by becoming a recluse and not bothering to speak with anyone or keep up with what is going on in the town. In turn, Miss Emily also rejects the taxes she owes, so by doing so, she has again shunned and rejected the town and its townspeople.
The property taxes represent CONSEQUENCES. When a person owns real estate, property taxes are due annually. The only way to avoid paying property taxes is to receive an official waiver from the governing body (the authorities).
Miss Emily has received a waiver from Colonel Sartoris. This waiver is seen by Miss Emily as being a perpetual waiver from the 'consequences' of owning property. No matter how long she owns her home, she expects to avoid the 'consequences' of paying her property taxes.
On a deeper level, she feels that she has been granted a waiver from the consequences of ALL her actions. Just as the Colonel has relieved her from the inevitable consequences of owning property, she feels that she has a perpetual 'waiver' to treat people in a cold, distant, standoff-ish manner, with no consequence. She feels that she is entitled to cohabitate with a man without facing any consequences, as she expresses to the Baptist minister. She also feels that she is free from the consequences of killing a man and preserving his body in her bedroom. In her mind, Colonel Sartoris has given her a 'waiver' for all of these actions.
Tobe was not granted a waiver by Colonel Sartoris, so he understands that there WILL be consquences for his and Miss Emily's actions. This is why he leaves the house immediately after her death and doesn't return.