Displaying both + and - for DC measurements distinguishes them from AC measurements which have no polarity. If I see 1.23 volts, is that positive DC or AC?
I suppose, but doesn't your meter already have an annunciatior for AC? The Fluke 87V (as well as my K2015) for example shows VDC and VAC to the right of the number, while the Keysight U1252B displays a ~ to the left of the number when in AC. (My K197 shows the tilde and AC when in AC.)
Older meters commonly lack additional indicators besides the digits, decimal points, and +/- sign.
My newer but old Beckman displays +/- in DC modes and a ~ in AC modes. My newer Tektronix DMM916 displays AC, DC, and AC+DC as needed and a - for negative but never a +. My older meters only have +/- to distinguish AC and DC like I described.
Of the above, only the DMM916 has leading zero suppression and when this is done, the - sign stays stuck at the left most side of the display.