Angled, not necessarily at exactly 45 degrees, but angled. Straight front foot will affect balance, weight transfer etc. I've seen fighters who keep their front foot straight, but haven't seen anybody teach it.
Just remember this: there are very few gospel truths in boxing. Learn what works best for you and use it. Know the benefits and trade offs of all techniques and use what is best for you.
Both. If your opponent is same stance (orthodox/southpaw) as you are, and you have your lead foot pointed at a slight angle inwards (it will be less than 45 degrees though, probably) it should also be pointed straight at the target (the opponent's centre line), because your opponent is standing at an angle, not square-on. The two are the same. If you actually pointed your lead foot straight from your own body it would usually be off target unless your opponent is usually standing square on, which is unlikely.
I would say angled. At least thats what my lead foot naturally does when I step forward with my jab anyway - so I figure its easy to just stay that way from the stance off rip.
I prefer angled way more than straight since it feels more natural. I don't really angle my foot 45 degrees though.