Linen cloth is as we know the first kind of cloth you will encounter in your adventures in World of Warcraft and it will start dropping from basically any level 5+ humanoid mob, up to about level 15-18 when you will start getting wool cloth instead . So if your aim is just linen, stick to mobs below level 15.
The reason you might want linen cloth may vary quite broadly, but most likely it's for either tailoring or first aid. Other professions like engineering does use some linen too, but not to the same extent. Anyways, you probably know what you need the cloth for, so I will stop ranting on with that.
As stated linen cloth drops from any humanoids between level 5 and 15. As you are leveling you will get a fair amount of cloth that you can either use right away or sell on the auction house to build your money. Grinding linen cloth for instance is much faster once you out level the mobs a bit. However, you might want to grind them for both experience and the cloth drops, which I can fully understand.
If you are grinding for both experience and drops, you want to find an area that has a high amount of mobs that respawn fast and then you start grinding away. There are a few mobs people usually suggest when it comes to linen grinding and one of the most common is Defias Trappers in Westfall if you're playing Alliance. For Horde players a good spot can be the Razormane mobs just north east of Crossroads.
People always answer about the ultimate grind spot for things in WoW and the above mentioned are just suggestions. In my opinion most mobs are more or less equally good as long as you stay in the right level range.
If you are a higher level charater that need to grind linen for any reason, be it first aid skilling, new tailoring profession, alt profession or just to sell on the AH you might want to go to Ragefire Chasm or The Deadmines depending on fact and such. You may have less luck with the linen in The Deadmines though since the mobs are higher level than in RFC. Also, in Ragefire Chasm, remember that you only need to kill the humanoids, the worms and elementals will not drop cloth for you.
Source by Anders Fredricsson