I am now a year older, wiser, and much more experienced in FOE. So, I would offer this reply.
Although, as you say, "sniping for contribution rewards" may cause the recipient to miss the critical timing of the "last" FP and the subsequent collection, I would argue that by contributing (or "sniping") many more FP's, I am actually affording the recipient the opportunity to avail him/herself with many more and much earlier collections, by accelerating the leveling of the GB. So, although this may be the accepted "etiquette" in the purest sense, it seems somewhat short-sighted by the recipient. Now, if the recipient is saving their GB for their guild/friends, that's a different argument and one that can be satisfied by mutual agreement on timing/scheduling/rotations, etc., which my guild does.
During the past four years, I confess that I have only committed this sin, twice. The second person to complain (a friend), has, several times in the past, swooped down on my GB's at the last moment, capturing the "pole position" and profiting to the tune of several hundred if not thousands of FPs', usually edging out my guild members. I have never been offended by this, in fact, I have been grateful, since it has allowed me to put my GB into service and start collecting sooner rather than later. And, since I don't consider FOE to be a "game of inches", I have to ask myself, what is the big deal? Can't we just enjoy the game without concocting a collection of unwritten chicken rules/taboos and tribal "etiquette"?
I have thoroughly enjoyed FOE during the past four years. I have been fortunate to be a member of an excellent guild with a nice group of people. But, there are two things that have brought me to the point of dropping the game (a) the parasitic looters, and (b) unnecessary recriminations from other members by daring to cross into their imaginary and protective bubbles.