It's a judgement call. Most people do consider it, but it really depends on what you want. Hell, you can wait to see what the first thing you run into is if you want.I'm thinking of doing this challenge on pearl. Just a question before it though.
In the very first battle against a lv.2 Starly in Lake verity, which comes out of the trees, not the grass, does it count of the first pokemon you see in Lake verity grass, or can you still catch the next pokemon you actually see in the grass there?
Moral of the story: It's your challenge, play it how you want.