When using a mouse it is possible to overshoot when moving towards a button, but if the button is at the edge of the display and the mouse is not capable of moving past that edge, then overshoot is impossible in that direction. Corners are even better because overshoot is impossible in two directions, hence the choice of the lower left corner for the 'Start' button in Windows.

With a touchscreen there is no on-screen cursor that has to be moved from its current location, which could be anywhere on the screen. Instead the user's finger is moving from somewhere above the screen down onto the desired button.

The edges still make attractive targets. If the button is at the bottom of the screen, then the user may touch the raised edge of the screen's housing and this ensures that the finger does not accidentally slide as the button is pressed.

When pressing buttons on the bottom of the screen the users hand will obscure a minimum of the display. On the other hand, buttons at the top can cause a lot of the display to be covered by the user's hand and arm, so the bottom edge is a more attractive home for buttons.

Similarly buttons on the right hand edge will not cause the users line of sight to be blocked as much as buttons on the left. This assumes a right handed user. While I would not advocate a design which heavily discriminates against left handed users, if your options are buttons on the left or buttons on the right, then it is sensible to go with the design choice that favors the majority.

It is sometimes visually more pleasing to keep the buttons a few pixels away from the edge of the display. Much of the benefit of placing the button near the edge is lost if it is possible for the user to place their finger up against the raised edge and effectively touch in between the button and the last pixel in the display.

You may think that the user's finger would be too large to cause a touch event in the last couple of pixels, but remember that there could be calibration errors which mean that the effective touch position is a few pixels away from the center of the finger. Alternatively a stylus may be used which would allow a touch event in that hard to reach edge of the display.

One nice compromise is available if the touch sensitive area of the button is greater than its visible area. It is then possible to display the button a couple of pixels from the edge of the display, but close enough that the touch sensitive area extends to the edge as seen in Figure 4 below.

For the mechanical design it is important that the raised edge is not so far from the display that there is a 'dead', non-touch sensitive, area between the display and the raised edge. For resistive touchscreens, this may be tricky to get right.

If the frame is actually touching the touchscreen then it is effectively generating touch events all around the edge of the touchscreen. This might not be noticeable at first, but pressure on the frame may vary with use, generating phantom touch events.

Over the edge
One anomaly of converting an analog touch signal into a location on screen is that it is possible to touch a position which is then converted to a pixel location which is not on the screen.

For example assume that a horizontal error is adding three pixels to every x location. If I have a screen of 1024x1024 pixels and I physically touch on location 1023, 100, then the software will detect that as location 1026, 100.

When I search my list of on-screen objects to find which one is activated, my software must tolerate a location such as this which is off the display. For similar reasons, a negative x or y value must be tolerated.

Consider an example where the touch sensitive area of a button stops at the edge of the display, at pixel 1023 (we will index the 1024 pixels from 0 to 1023). The touch event at 1026, 100 is beyond 1023 and therefore it is a miss.

It is most likely that the user was trying to touch the button that is place up against the edge of the display. Truncating the out-of-range position to the nearest edge would resolve this. So position 1026, 100 should be truncated to 1023, 100 before attempting to locate the touched object.

Be aware that some types of resistive touch screen are subject to non-linear characteristics and the non-linearity is most pronounced near the edges of the screen.

So you may place all the buttons near the edge to make them easier to touch, but then finding your biggest touch location errors are near the edges as well. So it is important to evaluate whether your touch panel suffers from this type of non-linearity.