Oh, the spies aren't a problem in my favorite spots... I listen real well and keep a close account of nearby allies. Its usually several other things that can present issues for me:
A quick win will stifle even the best sniper's killing streak. You can headshot everything that twitches, but if the round is over in 2 minutes, you won't make a record kill streak.
Also, something I've encountered is that you constantly have to advance if you want a decent run, then you usually cannot just camp your spot the whole round... a successful team will be advancing toward the final capture point (or the intelligence), and after a few minutes, your sniping spot may be seeing little to no action.
The spies do come, though

. More often than not, though... they are the enemy snipers who are getting frustrated and respawn as a spy to try and get rid of a threat.