Looks like I need to clarify what I mean by ‘all’.

At the top-most strata, ‘all’ would mean accepting and embracing the existence of the major ‘ancestral’ variants of Hokkien, i.e. 廈門 È-Múiⁿ, 漳州 Ciăng Cīu and 泉州 Cǔan Cīu, and their sub-dialects. From a pronunciation standpoint, that means, e.g. acknowledging that 火 can be he, hoe or hǝ, respectively. From a lexical standpoint, that means, e.g. acknowledging that 兩搗 nǑⁿ tâu and 兩擺 nng pâi mean the same thing. The parallels in British vs. American English would be the differences in pronouncing “can't/chance/past”, and knowing that the boot or trunk of the car mean the same thing. That is what I mean by ‘learn them all’, i.e. to be aware of the differences.
The next strata is where it gets complicated. The overseas variants of Hokkien would have undergone local influences (and here, even Taiwan is not spared, with the absorption of some Japanese loan-words, e.g. obasan for ‘old lady’). Where does one draw the line? This is very subjective. My personal background and bias makes me inclined to accept aokh1979's rule-of-thumb (ref: viewtopic.php?f=6&t=6276&p=29526#p29526) - whatever Malay loanwords that his grandmother knew and used, e.g. batu, tapi, are acceptable. Now, these I would accept at the local stratum of use, i.e. it is okay for Penangites to use these terminologies amongst themselves. But when speaking to non-Penangites, it is only proper that the common terms 石頭 chioq-thau and 但是 tan-si be used.
The final stratum is the most contentious, and the one that I personally do not subscribe to (but that’s just me), i.e. the injudicious borrowings from English, and the adulterations and simplifications that took place in the past three decades or so. So, boh-meaning, boh-fair, actually, horr, etc. are not acceptable to me. And especially not that ghastly word ‘kapster’, i.e. someone who kap-siau-kap-phi (chatterbox/tale-tattler)

This topic is, again, one that is difficult to answer. The reality is that languages, if left to take their natural course, are in a constant state of flux. Being a descriptive linguist, Sim would advocate a que sera sera point of view - which I respect, because he is embracing the reality of things. I, on the other hand, being the stubborn idealist, tend to adopt the prescriptive approach. Hokkien, with all its colourful variants, was at its best when our forefathers brought it across to Formosa and the South Sea, and before the buggers in the North went and flattened its birthplaces with Mandarin. Why spoil a good thing when it managed to escape China unscathed just in time? We should try to gel that linguistic snapshot in stasis, and not desecrate it any more than Mandarin is desecrating the other dialects in China today. But that’s me.
My Uthopian dream: aokh1979 and Andrew (representing Penang Sin-Kheq 新客 Hokkien), Sim (representing Penang Baba Hokkien), amhoanna (representing Taiwanese Hokkien), niuc (representing Bagansiapiapi Hokkien), siamiwako (representing Philippines Hokkien), Ah-bin (God, I think you know them all!) and all the rest of us, meet at a Hokkien fraternity gathering of sorts. We all speak our respective creoles of Hokkien, but respecting the unspoken rule of the lowest common denominator. And amidst the babble of the different creoles of Hokkien, we all understand each other well, and delightfully share in that fraternity.
And standing all around us are a bunch of Mandarin-only sad sods, scratching their heads in utter bewilderment.