While I agree with Stokemaveric that honesty is the best policy, simply 'keeping quiet' about something you're not actually asked about isn't dishonest, and it can't give grounds for dismissal at a later date.
Read the wording on each application form carefully and consider what the questions oblige you to divulge and what you can keep to yourself. If there's a specific list of medical conditions (e.g. diabetes, hypoglycaemia, etc) and yours is mentioned, you should declare it. Otherwise, if you genuinely see no reason why the condition should affect your work in the foreseeable future, don't bother mentioning it.
Again, with your dyslexia, only mention it if there's a specific question or if you can foresee a problem with your ability to do the work. (Incidentally, even allowing for the possibility that you've run your post through a spell checker, your dyslexia must be quite mild because your post makes far more sense than many of them of those which appear here on AB! Unless you're applying for a job as a proof-reader, I can see no reason why your dyslexia should affect your employment).
Chris