I have negative scores for bounces (e.g. -15 for two hard bounces in the last month). As this is a big indicator. I only set lead score to =0 when there is no activity for over 1 year. I also have a negative score for every month without acivity.
I'd recommend *increasing* the score if they already showed previous activity. When a prospect changes companies, the ensuing chaos can create both a sales opportunity at the old company and the new company. Most prospects find their inbox relatively empty at their new job, so this is an ideal time for sales to establish the relationship and learn how your product fits into the new hire's mandate. At the old company, its an opening to contact the manager and see how priorities will change.
We use a checkbox field called "doesn't work at this company anymore". This is manually determined by our inside sales. Checking it results in two tasks. 1) updating/finding the new mail address, 2) Who replaced the person.
For me increasing leads score makes no sense, because at that point in time the lead is already handled by sales. But still something worth thinking about. It's exactly these kind of questions someone should ask themselves when creating a lead scoring startegy.