I am sure that many people here no about this, but I still get tricked by it when using Google via google chrome, on occassions. For those who don't know about it, search on 'search history personalization' to find out how to turn it off, If you don't, when you are checking site rank (manually) the ranking of your sites will appear much higher than you actually are. Yes google is trying to help you find sites you already use, but I only just realized recently that it uses personalization whether you are signed into google or not. You have to turn off a button or clear the history/ cache. Anyway, this is old news, but hope it helps some newbs like me from falling into the trap of over estimating your rank and seo skills!
Yep, always make sure you have disabled web history & you are signed out of Google before checking your sites rankings. I sometimes make this mistake after checking my Google Analyitics, it keeps you signed in so when you return to Google the results are not acurate.
You can use another web browser to check page rank or other items. Plus, there is anonymous mode (EI - InPrivate, Chrome - Incognito, Firefox - Private Browsing).