For Square or other credit card swipers that fit a smartphone, you download an app. So, not too likely to do that on a client's phone. But, you could on your netbook, iPod, tablet, Nook, etc. Really only matters if you do returns at your clients' homes. I just type in the info in QB without a swiper for those who want to charge.
I pay for a data plan for my iPhone on AT&T. However, with other family members getting a base price of $9.99, we pay a great price for three phones.