Agricola Family Growth importance threshold
The sole* value of getting family growth is having extra action. Each round that you can get family growth earlier and is one extra action you can take. 

*Actually there are other reasons you might want to take family growth earlier - to block other people from taking it that round, or in order to not lose out on the three points for the family member at the end of the game, if other people take the space before you. 

When weighing up whether it's worth:

a) taking resources in order to build a room (ie. wood, reed)

b) building the room/family growth

You need to consider what that one extra turn is worth. 

For example:


If there is 6 wood there, is the one extra turn really worth the six wood? 
If there are five sheep there, is the one extra turn worth the five sheep?
If you can see that another player is short on food, and there's four food in the fishing pond, you might consider taking taking the food as an aggressive strategy that will hurt them despite them having an extra family member. 


Absolutely, generally speaking, growing your family early is a good a strategy, and extrapolating from that, that means taking the necessary resources (wood and reed) for yourself, or preventing others from getting resources is a good strategy. 

This does suggest that the 'take the single reed' could possibly be a reasonable strategy - both to acquire the resource for yourself, and to prevent others from grabbing the two reed the next round. 

Though - remember there is always the day labour in order to grab a single reed, and it would cripple your own game to try block both those spots each turn. 

Generally I don't favour taking a single reed - there are probably better options - such as plowing fields, or taking the wood. 

One better strategy, is if you are in last playing position, and there is still the one reed there, then you take starting player as your last move. This lets you grab the two reed first at the start of the next round.