i tether on my indulge and then on the spirit when i switched to it. Ultimate User :Agent Switcher " is an add-on or extension for chrome web browser, what it does, it tells to whatever website your opening/visiting what format to display, i.e phone mode , tablet mode, pc mode etc etc. Apparently , mpcs blocks all tethering traffic from the phone when is set to be displayed in PC format (user-agent-pc). However it is set by areas, some areas allow you to tether without the need to change the user-agent. In my own experience, at first i had to use a user-agent to connect my pc in tablet mode to my phone in order for metro pcs to believe my phone was doing the surfing on the internet. A few months ago i found out i have no need to change the user-agent any more.
Also if you are using the LG spirit, root and install recovery and install katana or icy spirit both rooms are the stock ICS and have the stock HotSpot app installed which works right out of the box with no need for you to try different apps from the play store.