I agree entirely - depends on the situation and how much of a rules lawyer you choose to be. If care and depth of character is excellent then I am far more amenable to alterations in the rules than if you are constantly looking for loopholes and scams. The rules are just a general guide line. Given that people have played characters for months without ever statting them at all is a testament to this.
Assassinates to take out sentries for example - if you want to be a real rules lawyer then you could suggest that each sentry was a seperate encounter. More than one strike to injure in a formal duel seems far more reasonable than using even a single one in a bar room brawl.
There are lots of things that rely heavily on GM's opinion, many religious activities rely on faith of a character not just the performance of that one act. Learning complex wizard spells etc. The more you roleplay it the more you get. The more you make it a simple game of numbers the less you get.