I guess this isn't a keeper or dynasty league then, if you're considering locking the rosters.
In the league I commish I try my hardest to do four things:
1. Maintain a fair balance of competition within the league
2. Keep owners from arguing with other owners. Mostly this is kept in check by enforcing #1.
3. Keep out of the way of how owners manage their teams with the exception of #1 and #2
4. Always stick to the rules you laid out in the beginning of the year, unless absolutely necessary to maintain #1 and #2, and sometimes #3.
In the case you're talking about, it essentially violates "rule" #3, but in order to keep #1 and #2 in check. If someone were to pick up a hot player off the waiver wire with nothing to play for, it could be assumed that another owner asked them to do it to block a team that they are up against in the upcoming week.
It might also violate rule #4 though, and this is where it gets tricky. Don't make this change because you have the #2 waiver claim and the #1 team is a non-playoff team, and you think the #1 team is going to use their waiver claim on <insert player that will make a difference> and block you from getting that player. It will look shady and ruin your league. I think sticking with #3 is the best method and keep the teams unlocked. If you have a problem, talk with the other owners before it becomes a problem.