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I was just referred to this question by graduate students that, in my opinion, were far too influenced by the answers. So let me start with two generic advises.

To the aspiring scientist: Don't assign too much weight to any answer on such matters, and don't assume that a small and highly non-random sample represents the common views among senior (or non-senior) people in the community. In general, think for yourself! See http://www.wisdom.weizmann.ac.il/~oded/advice.html for more details... To the senior scientist: Be careful about what you say, since it may be misinterpreted in harmful ways and/or have more impact than what you intend and/or perceive.

Re the discussion itself, I think that the idea that credits are non-monotone is not only utterly non-intuitive but also utterly wrong, and I am talking as a person who sat on numerous committees that took various career decisions. A person who has $X$ fundamental contributions, $Y$ important contributions, and $Z+1$ nice/legitimate contribution is ranked higher than one who has $(X,Y,Z)$, regardless of the numerical values of $X,Y,Z$ and assuming that quality captured by the $(X,Y,Z)$ triples is exactly the same. Trade-offs between different types is a different question, ditto re how much credit does each increase give...

In other words, for any set of works $S$ and any additional work $a$, the credit of $S \cup \{a\}$ is (strictly) bigger than to $S$ [i.e., strict monotonicity].

In my opinion, people who claim the opposite just assume that a larger number $Z$ implies a decrease in what the value of $X$ (or $Y$) could have been. But this assumption may be wrong and more importantly is irrelevant to the comparison at hand. That is, if you compare a case of $(X,Y,Z)$ to one of $(X,Y,Z+1)$, you must rule that the second person (called B) was able to meet the performance of the first (called A) although B also did another work of 3rd type; so B is clearly better. Indeed, you may think that B could have done better investing more energy in Type 1 (which is not always true - see below), but that's a comparison against an imaginary B, not against A. (And when you have a case of $(X,Y,Z)$ against $(X,Y,Z+10)$, the same holds is stronger terms.)

In addition, I think there is also a confusion between the works and the publications. If a work already exists in writing, and assuming that it has its merits, then it can only be advantageous to publish it in a adequate venue, where by adequate I mean one that is intended for works of this profile (wrt quality and scope - publication in a too prestigious conference may actually hurt, since it may generate some annoyance and even bad opinions re the author). But if one still has to develop a work from an initial idea (or "only" write it - which always involves some more research...), then one may consider the trade-off between the amount of time required versus the importance of the work.

Finally, as I hinted above, it is not clear that one is better off aiming all the time at Type 1 (i.e., fundamental work). Firstly, this is infeasible and thus problematic/harmful. Secondly, and more importantly, one is always better off following the inherent logic of his/her own interests and ideas/feelings, and aiming to do as well as possible. See more in the aforementioned webpage.

Oded Goldreich