I have a lot of dark brown in my artwork designed in photoshop, which is C73-M62-Y98-K73. Does it matter that the sum is over 300%? Should I be limiting my colours to a certain percentage?
What you are referring to is called TAC or Total Area Coverage; it is usually defined by your printer. 300% is a good personal limit, as many range from around 240 to 320. If it is personal work it is completely down to your judgement.
Thoose maximum values depend on the standard you are using. The SWOP V2 recomends a maximum value of 300%, But the Fogra 39 accept 330%.
Your values are at 306% so it is ok.
How theese big swaches of color are handled depends on the climate conditions on the print house, the type of inks used, etc. But they can implement measures to avoid this color to stain the other sheets of paper, like not staking too much paper on top of each other, using a warmer environment, a little slower passes, etc.