I have six days left on my billing cycle and I just hit 5.29 GB...I don't tether but I do stream Pandora a lot. It is my main internet device.
Not trying to freak you out or anything here, but if I am correct about the $0.25 per mb overage...your about $80 in at this point.
I've never had overages for data usage so I have no idea if calling them ahead of time will help. But I suggest you do so, ESPECIALLY before your billing cycle ends.
I've had them help with minute overages by letting me upgrade my minutes to one that covers my usage and then post dating it. You still pay a little more than the one month fee because of the pro-rated protion for a partial month or something...hard to figure out, but its better than a $600 phone bill.
What I do remember though is that if you wait till your billing cycle is over, they can't help you, and your stuck with the bill.