Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $78.56   3.773%   $81.5241   $81.52   $81.50   $81.50  
  Purchase #2 $109.90   8.153%   $118.8601   $118.86   $118.85   $118.90  
  Purchase #3 $11.29   10.807%   $12.5101   $12.51   $12.50   $12.50  
  Purchase #4 $135.54   9.653%   $148.6237   $148.62   $148.60   $148.60  
  Purchase #5 $65.36   6.573%   $69.6561   $69.66   $69.65   $69.70  
  Purchase #6 $180.66   8.325%   $195.6999   $195.70   $195.70   $195.70  
  Purchase #7 $208.72   13.231%   $236.3357   $236.34   $236.35   $236.30  
  7 purchase totals = $790.03       $863.2097   $863.21   $863.15   $863.20