Where to define nomenclature in a proposal or dissertation?
I personally think the biggest consideration is how often the term or symbol is used. If it's used a lot, then a clear definition at the start is probably the best. The reader then knows where to look if they're unsure of the meaning rather than having to find the first place it's used, and it would be ridiculous to keep defining it every time you use it.

For things that are just used once, it may be more readable to define it at the point of use. The definition is only relevant for that little portion, so splitting the definition and use makes things, as you say, too much of an effort for the reader.

The question then is where to draw the line; at what point does a rarely used term become common enough to warrant being included in the nomenclature section? This is, I think, becomes a matter of preference. Personally, if a term were used in more than one section, I'd define it at the start. Though, if it really is only used twice, perhaps an in-place definition and a "recall that we define..." kind of sentence would read more smoothly.