With Protospiel approaching and me hurrying to preparing my prototypes for testing I have been thinking allot about how polished does it need to be before I put it in front of tetsers.
I basically have two levels.
Testing with friends: These can be pretty raw since I have done the same for them.
Testing with others: I need to know that the game is solid at its core.
How tight does your design have to be before you have it tested?
I do all my playtesting by myself at first until I have something that I can play through and not have to stop and fix. After I have that, I'll get my wife and close friends and family together to see how it does with multiple players and people who don't know the game - at this point I'm also beginning to design the rulebook using their questions and learning process. Galaxy Corporate went through... nine versions, I think, while testing with my friends.
I have since put it out as a PnP game, but I still consider it the "beta" version since I'm now in the process of getting the physical design together for printing. I'm still testing, but at this point I'm looking for reactions. What's fun, what's not fun, how many little tokens are being used, what's the pace of the game, etc. The game itself is solid and I'm satisfied with it, but there's still potential to tweak things while it's not set in cardboard yet. This is the stage at which I'll let the public play with the game - testing to see how "fiddly" it is, checking all the superficial aspects of it.
Some people like to grab random strangers and do blind testing almost immediately. Myself, I'd like to just laugh at my mistakes over beer with my friends. And they have done much laughing at my mistakes. ;)
Imo, when you're sending it to someone you don't have a physical contact with (you can't explain the rules yourself cause you're simply not there physically) you need to make the rules as tight and clear as possible.
Basically, you need to write down the rules as you would for a mass-market production game. Yes, it is hard, I know. Take your time and make an effort to create cheat-sheets with reminders for the players as well as examples of play.
The one thing that you can do in prototype phase that you cannot do in production is that you can add notes with your suggestions on possible rule changes. This might present you as an amiable chap who is open to suggestions and who places the game rather than his ego to the forefront... but don't over do it. You don't want to look indecisive or lazy. Strive for the balance. Make rules as clear and definite as possible, but do point out a few "negotionable areas" that you admit you're not sure about and would like some creative feedback on.
If you make it clear that it is the quality of the game and the players' enjoyment that is the most important to you, as well as that you do have something personal and unique to offer, you'll do great.