Is the shape of (SMD) pads relevant?
For the most part, no.  It doesn't matter.  Or, to put it a different way:  For about 95% of the designs out there, the shape of the pad does not matter.  There are some things to consider, however, before just assuming that your pads are correct:


BGA's need round pads of a very specific size.
The pad has to be big enough to hold enough solder paste to do the job.  There needs to be enough solder to create a proper fillet for reliable soldering.  The size of the stencil, along with the size of the pad, will determine how much solder is available for that pin.
Some RF guy is going to say that the shape matters.  I am not an RF guy.  But I do design digital circuits with 2.5+ GHz signals and the pads don't matter for me.
Sometimes the shape of the pad is used to identify Pin 1.  Square pad for pin 1, rounded pad for the other pins.
One might argue that pads with rounded corners are less likely to de-laminate.  I have no idea if this is true or not.  But if it is true, it can only make a difference if there is a lot of rework being done on the PCB.  For normal automated assembly of PCB's, this doesn't seem to matter.
If the part manufacturer recommends a specific pad size/shape, then you better follow what they say.
Square pads will get slightly rounded off in the etching process.  For medium to large pads you'll never notice.  For small pads you might care.  I almost never care.