We're trading off avaiability, consistency and complexity. To address the last question first: Does this matter? Yes very much! The choices concerning how your data is to be managed is absolutely fundamental, and there's no "Best Practice" dodging the decisions. You need to understand your particular requirements.
There's a fundamental tension:
One copy: consistency is easy, but if it happens to be down everybody is out of the water, and if people are remote then may pay horrid communication costs. Bring portable devices, which may need to operate disconenected, into the picture and one copy won't cut it.
Master Slave: consistency is not too difficult because each piece of data has exactly one owning master. But then what do you do if you can't see that master, some kind of postponed work is needed.
Master-Master: well if you can make it work then it seems to offer everything, no single point of failure, everyone can work all the time. Trouble is it very hard to preserve absolute consistency. See the wikipedia article for more.
Wikipedia seems to have a nice summary of the advantages and disadvantages
Advantages
If one master fails, other masters will continue to update the
database.
Masters can be located in several physical sites i.e.
distributed across the network.
Disadvantages
Most multi-master replication systems are only loosely consistent,
i.e. lazy and asynchronous, violating ACID properties.
Eager replication systems are complex and introduce some
communication latency.
Issues such as conflict resolution can become intractable as
the number of nodes involved rises and the required latency decreases.