The answer, as usual, is "it depends". It depends on what kind of shots you hope to get, and how you plan to use your camera. Flash is beneficial for shots where there is not enough ambient light and the area to be lit is fairly small. It is also very useful when you need fill or to overcome extremely backlit subjects, such as when you must face the sun or a very bright window.
Flash can be useful indoors, in churches, museums and other such places. However, a flash will do very little to light an entire church, but might be useful for a statue or a few elements of detail you wish to capture. However, many places forbid flashes, as the cumulative light damages artifacts and artwork. So, instead of a flash for indoor travel photography, you would be better off with a tripod, and simply setting the camera on a slower shutter speed to capture your subject.
For fill light, a flash is really useful. Luckily, you have a built-in flash on your t3i, and this works reasonably well for fill. However, it is likely that the 55-250 lens might be long enough to block some of the flash. The 430EX will provide more light, and throw it further than your on-board, and it is positioned higher, so it will likely clear the lens. It also offers a separate focus light, that really aids in getting focus in low light situations. But frankly, a pocket flashlight, shining on your subject during focus does the same. For fill light, a reflector is a must have, but any white object for reflecting works well in a pinch: t-shirt, magazine, piece of paper.
So, my recommendation is that for travel photography, forget the flash. Buy a tripod instead, put a small flashlight in your pocket, and carry something to use as a reflector.