My friends and I have played three or four games of Carcassonne with varying winners, and fortunately we're all about the same experience with board games in general.
I'm beginning to find, however, that our games are being decided by the farm at the end. It is common for thirty or forty points to be decided at the very end. With this in mind, should I start shifting my strategy toward keeping only one or two followers open for quick road/city/abbey completions and lay down more farmers?
It sounds like you are pretty new to Carcassonne. It's pretty easy to start playing and have your group think that nearly everything should be finished and the board should look nearly square at the end without many blank holes in the grid.
If this sounds familiar, then yes, there are likely to be big farms and its a good idea to get in early on the farm.
You could change your style though. I view every meeple played by my opponents as something to be attacked and stranded!
When an opponent plays a farmer, that meeple isn't moving ever again. You need a good reason to extend that farm in any way. Play around it. The next road juncture you play should be positioned to cut off the farm. Do not play city tiles on the farm. Just leave it alone!
I think the answer to this question really depends on the state of the board.
Generally, if there are large farms with lots of smaller cities, then of course you should shift towards laying down farmers early and claim farming territory.
But if there are a few large, unfinished cities which look hopeless, or lots of little farms broken by many roads, then it doesn't make sense to lay down the farmers early.
If a farm is going to score 9+ points easily, then "quick" cloister/road/city completions are not going going to make up the difference. On the other hand, meeple management is important — if you tie up all your meeples too early, then you could put yourself at a disadvantage.
Beware of laying too many farmers too early on farms that are quite close to each other, lest cunning opponents join up all your farms so you end up with one farm, which may be quite large, but only scores you the points for the cities once, not for the multiple farmers.
AND beware of farm 'thieves' - my friend is a master of this - he'll wait until the last 20 or so tiles, then start adding farmers on tiny bits of what look like non-point-worthy sections of field, and then on subsequent moves join them up to MY farms - usually with a 2-point meeple (from Inns & Cathedrals expansion) or actually two or more meeple, thus depriving me of my huge farm score I'd been building up the whole game. The swine! :)
EDIT: I released I didn't actually answer the question posed. I usually use around 3 meeple for farmers during a game, then try deploying any and all remaining meeple as farmers near game end to grab last minute points from any remaining un-farmed fields. Deploying too many farmers too early on can lead to the above noted things happening, by which time you have little way of gaining any foothold back!
Farmers are a great way to swing the board and win a game, but you have to be really careful because your meeples are essentially trapped. The answer, as ever, is it depends. But there are certain things you should avoid:
Do not overcommit meeples to farming. They become trapped and you'll become frozen from scoring. This is especially important later in the game.
Ask yourself if committing a farmer is worth it. It's easy to see if the board board will turn into a cluster of small cities, or some giant disjointed cities. I prefer to observe this pattern before I commit. Three or six points early in the game are not worth losing my meeple. I can use them to score similar amounts while retaining their use.
Don't rush. Joining farms belonging to opponents is easy with careful planning. Keep an eye out for curved road tiles and green patches on city tiles. I've seen games won and lost with the last tile on too many occasions.
There's no such thing as a useless tile. Tiles that seem useless can be used defensively to prevent people from taking over or infiltrating your farms (or other features).
Advanced players prefer to score small quick gains while holding on to their meeples. At the end of the game you'll usually see a blitz for small farms and other features as scoring opportunities diminish. I play regularly against players with circa 1800 ELO, and I've observed this pattern.
In the end, the biggest mistake newbies make is they over commit meeples to farming, or completely overlook it. The trick is finding the right balance and reading the board well. It's about using your meeples efficiently, and this is very situational, but the above guidelines should stand.
