Dthrow is always escapable if you DI away. Maybe there are some exceptions with some fatter characters, but I wouldn't know. If you're high enough percentage that you can't be regrabbed and you aren't floaty, a Fair will probably kill you anyways.
Bthrow definitely seems to be escapable at a lower percent than Dthrow, but it can also be comboed out of in that situation. DIing toward them or not at all will not allow for a regrab, which makes it not a 50/50
Since DIing for one can lead into a regrab for the other and you have to predict them to escape, barring the exceptions above you do have to guess to get out. It should be noted however that Dthrow is as a whole the better throw, leading to tech chases even if they don't regrab. As a result, they're much more likely to Dthrow. That means that you should be DIing away most of the time. Just be wary that a bad guess into a back throw can lead to a killing Usmash on some characters. At the end of the day you should be mixing it up, but if they seem to have a tendency towad doing one you should obviously DI that on correctly more often.