Misia,
Whether you get in any trouble is area specific. You have two options. Do Call the county/City/Township offices and find out if they have anything outlawing the signs. The problem with this is they probably won't know (especially if not a bigger city) and will just say no, you can do it. Option 2 is put up your signs (yourself or paying someone) and hope for the best. I have heard of some investors who have had municipalities come after them, but can't point to a specific example so there is some risk in this (make your own decision). But I would rather pretend I didn't know any better and beg forgiveness if they got mad and said I couldn't do it.
I put them out in a town I won't name and the signs disappeared quickly (think suburbia/rural in areas). Every week, kept putting them up (and not taking them down). I put them up on a Friday evening/night) and soon the city started calling me and threatening me and telling me all city employees where instructed to take down any signs they come across going about there business.
The signs were on power line poles only and in easement areas next to Realtor signs. I haven't tried private property. They called me every time they found more signs. They never gave me a ticket or fine, but made it clear that this was not going to happen in this area. I averaged putting out about 50 signs every week or two until the city started calling. They called me for a few more weeks whenever they found a sign and I always said I had not put out any more and that they are just finding some stragglers (which is true). So in a few weeks the calls ended and that was the end of it.
My point is I prefer option 2 and taking my chances and accepting I may get some heat if the municipality wants to pretend they are tough and powerful. I don't live in that area any longer and I am willing to accept the risk of a possible fine, however improbable. But only you can make that decision for yourself. I haven't used signs much since except to sell. I personally didn't get a ton of calls but my signs didn't last very long so I didn't do a long enough test to be significant IMHO. But I do need to try them again. If you can find an area where they last a while, I think you would be crazy not to use some. Every little bit helps as there is no such thing as too much marketing IMHO.
Good investing
Mike C