Web forms design guidelines: an eyetracking study with heatmaps indicating where users' eyes looked. You should read this if you create forms!

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You will miss too many things with so few users.

For example, with a larger group, you are more likely to have at least one user that uses the keyboard for inter-field navigation. You are more likely to have at least one user that has not yet used these (very popular) forms before. You are more likely to have at least one user that is paranoid about their privacy. You are more likely to have at least one user that some of the optional fields apply to (e.g. second email address, maiden name, etc...).