Is there a standard for schematic power symbols?
After a quick check of IEEE Std 315-1975 Graphic Symbols for Electrical and Electronics Diagrams (Including Reference Designation Letters), the following seems applicable:

3.9.3 Common connections
Conducting connections made to one another.
All like-designated points are connected.

NOTE — 3.9.3A: The asterisk is not part of the symbol. 
  Identifying values, letters, numbers, or marks shall
  replace the asterisk. For the triangular symbol, this
  identification shall be placed within the triangle
  or, if essential for legibility, adjacent to the
  triangle.


3.9.3.1 "Specific potential difference" seems directly applicable. Basically, you place a descriptive "designation" label at the end of the line segment that has a "specific potential difference with respect to a potential reference level."

---- *  (See note 3.9.3A)


For example,

[+5V_power_supply_circuit]---- +5V

      +5V
       |
[CIRCUIT_ELEMENT]


That's it. No "T", no "bubble", etc. And for what it's worth, I often see this method used by companies like Keysight Technologies, Tektronix, Fluke, etc. in the service manuals they publish for their test equipment products. For example, look up the 34401A digital multimeter on Keysight's website, and under the "Document Library" tab you'll find a link for the "Service Guide" manual for that DMM. The schematics are at the very end of that manual. (You might need to create a user account on Keysight's website to access their manuals. That's so annoying...) (n.b. Hewlett-Packard == Agilent == Keysight)

(n.b. Note 3.9.3A mentions "marks". That would permit the use of "T" symbols, bubbles, etc. at the end of the line segment.)

One final comment. The main thing here is clarity and consistency. Some EDA programs define a "T" symbol, or a bubble symbol, or some other symbol for use with power connections. Most electronics techs and electrical engineers clearly understand what these symbols mean if the symbol is accompanied with a meaningful label (e.g., +5V); so don't shy away from using these symbols if that's the symbology that's available to you or that you prefer. Just be consistent: prefer to use only the IEEE's style, or only "T" symbols, or only bubbles, etc. throughout the entire schematic; avoid using a confusing mix of different types of power distribution symbols on the schematic.