theres a little more to it than that - Ive read several guides and collected data my self (and got my SAT scores back yesterday... <omg>790 in math
there are what I call "squares" which are basically, squares that are a certain distance from your player. I denote them as n-squares, where n is their distance from you. While using the poke radar, you only need to care about these:
4321u1234...........the u in the middle is you, the numbers are part of the n-squares
I have never seen any patch of grass wiggle outside of the 4-square. Also, If you pay close enough attention, you will notice that only 1 patch of grass wiggles in each n-square up to 4. Sometimes, an n-square wont have a wiggly patch, this happens randomly and when the one the game decided would wiggle was outside of the grass. Thus - your chain will not always break on edges, but there is a possibility that if you are on an edge (especially a corner) all of the wiggly patches will lie outside of the grass, resulting in "the grassy patch remained quiet..." and breaking your chain.
43333xxxx..........numbers represent grass
43222xx$x..........x's represent non-grass
43211xxxx..........$'s represent wiggly spots
xxxxxxxxx...........The grassy patch remained quiet...
There is no place where there is a guaranteed continuance (?) of your chain, but in the 4-square the chance is reasonable large. If I follow only the 4-square patches and reset when they dont appear, I can reach a 20 chain without much trouble (except when I forget to reapply repels
Also, I have not experimented on whether using only the same kind of patch affects your chances, but I bet it does. Also- I read somewhere that walking into a patch with another patch immediately next to it is bad, but I have AS OF YET FOUND NO EVIDENCE TO CONFIRM THIS.