It's a little bit more complicated, with a lot of history and hardware changes over time. So, the full answer is a bit tricky. Also, you actually have two questions... PT's on computers, and LR stuff. However, the easy solution in most cases is to do what wv2win said and use a 2 amp or better wall-wart.
Now, that said... as a generalization, if you're using a 3.x volt PT that HAS AN INLINE BATTERY you are vaping off the battery...not really the USB. Any USB device can fail. And with e-cigs, USB cord shorts seem all too frequent. However, a PT with battery only trickle charges the battery from the USB, so it can do that from a computer port. That said, why tie up a computer port with an e-cig? Why risk frying a port when a wall-wart is $5.00?
However, they also have 5 volt "direct" PTs. Those don't have a battery and draw strait from the USB connector. Too much for a computer...even without using LR (which is too hot at 5v). You can do it for a while with SR stuff on newer 3.0 ports but it's risky and could fry them. At best, the voltage will drop due to low amps and stress the heck out of the port. Now add DCC's into the mix and it's pure trouble.
The "history" part of the LR equation for many e-cigs includes (?rumored?) frying of mosfets (circuits) on the e-cig due to overamping... PT or no PT. This is rumored to be resolved with most models. However, occasional reports of premature failure of the e-cig still happen. OTOH, there's tons of users that swear they have used LR stuff on their eGo type battery for a long time and never had an issue. Others suspect that they can either fail randomly, fail based on vaping habits with LR, or fail due to an internal short when the LR atty/carto gives out. Note that this doesn't normally apply to mechanical mods (i.e. box mods with a 3.7 volt battery) since they often don't employ a mosfet and don't have much if any circuitry to fry (other than the protection circuit in the swappable battery).
This is a bit paraphrased from memory and gleaning a lot of posts. However, it should get you thinking.
