Which players to save, at certain points in the season, is an interesting question. If it is possible to save players at no cost to the national teams (in terms of competitiveness in non-scrimmages), then I think it makes sense to save the best young players possible for those weeks (including longshot types). If a valuable season-long roster spot is to be tied up on a prospect, then who to save becomes a tougher question.
Potential is a limiting factor on development, and looking at the current NT scene (including developing players in the pipeline), I think that Superstar+ potential will be needed everywhere in the future. I could even see MVP+ potential being needed almost everywhere in the near-ish future, depending on how some key players develop.
For the U21 team, though, you can get very effective players with allstar or perennial allstar potential, especially at the guard spots. For big men, I think that U21 quality big men really should have perennial allstar+ potential, although allstars can contribute to the Canadian team (and have in the past).
Overall, I would say that I would want Superstar+ potential for NT track guys, and P.Allstar+ potential for most U21 players, although a very high quality allstar could also be worth saving (for U21 purposes). I can't really say what salary cutoffs I would rely on, as it really depends on the particular player. If it turns out that there are very high potential players, that also have good skills, then I would prefer to use spots to save those players. However, if a really good U21 only type player is retiring, then I think it makes sense to consider a player like that as well. I wouldn't rule them out, at least.