It's less important in DFRPG than it may be in other games. Or, perhaps more correctly, one RAW comment makes the rest of the book...fuzzier: "...intent precedes mechanics." (YS308 under Adjudicating the Rules)
As Bruce mentioned, flexibility is expected. Not allowing the mechanics to limit options is also codified. 
Yes, but it is always important, imho, for players to have a good idea about how reality in a game works. Hence them understanding the rules (including any house rules or GM interpretations) is important. It helps them make better decisions for their characters and prevents those horrible moments of conflict when there's a huge disagreement over how game reality works -- one can't entirely eliminate such problems, but one can reduce how often they come up.
The adjudicating the rules bit is, in itself, an important rule. However, consider the many ways one can handle initiative. Some of them might allow a slow character to interrupt a faster/supernatural/mythic speed NPC and others might not. Which one you are using can indeed matter a great deal if a player decides to delay under one understanding of the rules only to feel that he got screwed over because the GM has another. The "intent matters more than mechanics" doesn't help here if the GM thinks the player intended to do something they feel is impossible. Problems here are best avoided by the GM having a careful understanding of the rules, being forgiving of player misconceptions, and the group working to make sure everyone has a fairly good consensus on how game reality works, imho.
