Not realy, but it's a definate advantage, on thing is that if you export from Zbrush to maya you have to flip the texture (as Z has a different coordiante system (think that the y axis is down rarther than up if I remember rightly) You can do it in maya if you needet too.

Also think of if you texture something in Z brush and when its in maya it dosent really look how you want, using PS you can just simply alter the colour balance etc to suit.

Also Photoshops a lot easier when texturing ciritan object.

all in all I would learn is as it's got lots of fundamental things that you will probably need.

On a side note you dont have to use Photoshop, any other image manipulating tool will do the job, I would reccommend GIMP, its free and does all the same things that Photoshop does, and possibly more.