I've crossed more fences than I care to remember. I can almost always find a spot where hundreds of others have already crossed. Dweste, in your case, I think the advise to go round or stay on sidewalks is the wisest. If you are encountering many security fences, you aren't really in the "country".

I don't think I'd even contemplate crossing a security fence unless I had an imminent threat to life and limb right behind me.

Another point about travelling unseen: Do it at night with a quarter moon. Amazing how well your night vision adjusts after 45 minutes. Hole up during the day, move at night. Just like the recon dudes in the marines.