words where it is important to distinguish between the neutral tone and the citation tone I'm working through the HSK vocab lists, and sometimes I find it hard to remember if a character should be pronounced with the neutral tone or with the citation tone. 
It would help if I knew when it's important, and when it isn't; if I knew when "getting it wrong" could lead to confusion. 
For instance, I believe 结实 can be pronounced jie1shi5 or jie1shi2, with two different meanings depending on whether 实 is stressed or not ("solid; sturdy" or "to bear fruit"), so here the neutral tone seems to be important. Whereas with 姑娘, it doesn't seem to matter whether 娘 is pronounced niang2 or niang5. 
Still, I believe there are also cases where getting it wrong with using/not using the neutral tone could make it harder to be understood, even if there is no ambiguity. 

Examples:


东边,西边 and 里边: it seems most dictionaries give the second character as
bian5 rather than bian1. But if you were to say li3bian1 rather than
li3bian5, could that make it harder to be understood?  
欺负 to bully: qi1fu5, not qi1fu4    
时候 shi2hou5, not shi2hou4
云头: yun2tou2, not yun2tou5; cloud. Here there seem to be more words ending in 头 that are pronounced xxxtou2 than words pronounced xxxtou5.


The section "Second-syllable Stress" in this blog post has some bearing on this topic. 

Perhaps what I'm looking for is guidance about characters commonly used as suffixes with the neutral tone, but that also can occur as suffixes with one of the other tones. For example, 子,头 and 边. Also perhaps there are some rules of thumb or some "gotchas" that could be helpful. 

I'm aware this topic is related to variant pronounciations, regional accents and dialects; it seems to be quite tricky.
Zhang and Yang (2007:84-89) describe several instances where pronunciations in the neutral tone and the citation tone of a word may coexist:

When contrasting polysemy (辨義). Usually, the neutral tone marks informality or figurative speech.


東西 dōng xi means 'thing; item', but dōng xī means literally the East and West (the cardinal points, or more broadly speaking, the Orient and the Occident).

買賣 mǎi mai means informally 'a transaction', but mǎi mài means 'the collective act of buying and selling'.



When contrasting 'parts of speech' (詞性, lit. the nature of the word). These words are semantically similar (or related). In general, the citation tone is regained when a noun becomes a verb, an intransitive verb becomes transitive, or when the word becomes part of a compound word (e.g., a compound noun, or an idiom). That is to say, the citation tone is needed for complex, derived instances.


差使 chāi shi as a noun means 'an errand runner', but chāi shǐ as a verb means 'to assign (sb. sth.)'.
避諱 (the verb 'to taboo') pronounced bì hui is intransitive, but transitive when pronounced bì huǐ, as in 有所避諱 (having something to apply a taboo on), 避諱名字 (to taboo a name).
利害 (the noun/adjective 'profits and losses') pronounced lì hai is a stand-alone word, but is a constituent of compound words like 利害關係 ('an interest', here the meaning inclines to 利, see 偏義複詞) when pronounced lì hài.


When contrasting formality. These words are semantically identical. The neutral tone is used frequently in colloquial settings.


聰明 (the adjective 'clever'): cōng ming (colloquial) vs cōng míng (formal).
打扮 (the noun 'manner of dressing'): dǎ ban (colloquial noun) vs dǎ bàn (formal noun; or the verb 'to dress up' by rule 2.).


As a result of dialectal differences. In mainland Chinese, some words must be pronounced in neutral tones. In Taiwanese Chinese, however, neutral tones are scarce.


Certain suffixes ～子, ～頭, ～巴 etc.
Directional complements ～上, ～下, ～去 etc.
Particles ～的, ～着, ～一～ etc.


Quasi-neutral tones are permissible in speech to contrast intonation. These words have weakened tones in the face of other stressed components in a sentence. However, they are not documented neutral tones in authoritative exhaustive lists.


家中光景很是慘淡。(hěn shì &gt; hěn shi): in the presence of the stressed adverb 很 indicating the severe extent of poverty, 是 should be read lightly.
你就在此地，不要走動。(zài &gt; zai): in the presence of the stressed adverb 就 indicating the immovability of the listener, 在 should be read lightly.
還說呢，快開始吧。(hái &gt; hai, kuài &gt; kuai): in the presence of the stressed verbs 說 and 開始, indicating the speaker's impatience, adverbs 還 and 快 should be read lightly.

Reference: Zhang Bennan &amp; Yang Ruowei (2007). [普通話連讀音變]. Hong Kong: Commercial Press.