I track them in a separate spreadsheet and I assign conservative values to them and to calculate a redemption value. It helps me evaluate which bonuses to pursue and make choices about spend or burn. I have a separate spreadsheet where I track my net worth. I haven't added the miles to that spreadsheet, but in concept I would have no objection to doing so. Its an intangible asset that is assigned to you. I wouldn't use it for a bank loan, but if you own anything but market priced stocks and cash there are estimates in play in calculating net worth (e.g. house value, cars, collectibles).

Mint is okay for day-to-day tracking, but my experience is the historical data is prone to gremlins, so I keep a spreadsheet in parallel.