Hi Shani, buffers vary in their ability to maintain physiological pH. For example Tris is a crappy buffer between pH 7 - 7.5.
Tissue culture media use HEPES or carbonate to maintain physiological pH. Having phenol red in the media should tell you whether the pH is above or below physiological condition.
I use both to culture my cells. And I have to say, that I've never noticed any differences in the pH (by looking to the color of the media). Temp., cell density, and CO2 levels would influence way more the pH