Re: sink tip casting help
It (whatever working line you have peeled off the reel) will tangle around your feet and/or debris when wading, but it can be contained more easily in a boat. For things like surf fishing with a full sink line you basically HAVE to use a stripping basket to contain your line (because it sinks and the waves knock it around like crazy). With a floating line or a sink tip line that has a floating running (back) portion, this isn't so much a problem....though you certainly can get "floating tangles" if you don't manage your line well.
That said, I sometimes use a full sink line when wading and just use a stripping basket to manage line even in lower flow areas. For still water I sometimes don't use it at all -- just am more careful about how much "working line" I have out.
It basically just depends on where you are fishing and where the fish are located. You can always experiment with different line types on your waters and see what works best. I generally use a floating line for any floating fly pattern (obviously), but also weighted nymphs and some streamers if the fish are not holding too deeply. The problem is that the line floats on the surface and when you retrieve it tends to pull the fly "upward" to the surface, which isn't ideal sometimes. If I need to get streamers down deeper than about 4-5' I tend to prefer a full sinking line on still/slack water areas just to keep it at the right depth during the retrieve. Sink tip lines are useful for areas with decent flow (rivers) where you want some extra depth but still need the floating back portion of fly line to make mends, etc. I don't use sink tips very much because I don't fish rivers with decent current very much.
You'll also see mention of the "intermediate" fly lines out there. They're just a full sinking line that sinks very slowly -- good for shallow and medium depths where you want the fly to stay under during retrieves. A lot of people use them in the surf to get under the waves, it all depends on the location and conditions.