In the ARINC specification, does "sig bits" include the sign bit?
In A429 BNR words bit 29 is the sign bit.  Individual label descriptions are not entirely consistent in that many will identify bit 29 as part of the sig bits field.
In any case it's important to look at the details of a specific label, especially the range, MSB, and LSB values.  Some are always positive (such as airspeed) and so the sign bit is assumed to be positive and bit 29 can be the MSB.
One common factor is that the majority of BNR labels are coded in 2's-complement binary.  In this case, the MSB is half the range and each of the following bits is half the value of the previous. Example: For a "heading" range of 0 to 359, the MSB (bit 29) is 180, the next is 90, followed by 45, 22.5, 11.25, etc. Alternately, you can declare bit 29 the sign bit and the range becomes -180 to +179. 
So you'll note that in 2's-complement encoding, it doesn't matter if you consider bit 29 as an MSB or a sign bit.  The difference is that if you call it MSB, the range is 0 to 359.  If you call it a sign bit, the range is -180 to +179.
You can prove this to yourself by declaring bit 29 as a sign bit and code -135 degrees.  Then declare bit 29 a MSB value of 180 and code 225 degrees. The coding should be the same.