Voiced bilabial fricative vs. voiced bilabial plosive

Voiced bilabial fricative vs. voiced bilabial plosive
 

Couldn't quite figure out where to put this, there's no pronunciation board.

I was once under the impression that all b's and v's represented the voiced bilabial fricative in Spanish, but I've come to understand that the voiced bilabial plosive also exists in Spanish, and that sometimes even the voiced labiodental fricative can make an appearance as an allophone of the voiceless labiodental fricative before other voiced consonants.

My questions are, are there any guidelines for deciding when to use a plosive or a fricative when encountering the letters b or v? Do I need to memorize IPA representations of every word containing a b or a v? Are these just things that I'll have to grasp intuitively as part of my accent, much as I do with vowel sounds?

The simple, linguistics-aside approach that I always recommend is mimic what you hear.
When in Rome, ...
When in Venezuela, ...

There are variations in pronunciation throughout the world.

There are regions and people who believe it's more educated to make a difference when pronouncing these two sounds, despite the fact that officially there is no difference between b and v in Spanish.
Most of the people who pronounce both sounds differently is because they speak another language or because old school teachers taught them so. :)

I was under the impression that all bs and vs in Spanish should be plosives, at least in the peninsular prestige accent. Your question has provoked me to look for a source, and Wikipedia's IPA for Spanish seems to extrapolate to a general rule (violated by one of the eight examples, but I suspect that to be in error) that a b or v in a stressed syllable is plosive and in an unstressed syllable is fricative.

Gracias a todos, todos sus respuestas era muy útil.

Algunas correcciones. :)

And here I was just coming back wondering if I made the possessive adjective agree in number with the noun. Thanks, Rusty!