A big factor in how much you pay for every click is quality score. The likely biggest factor in quality score is your relative click through rate for the position (i.e. Google wants to show the ad that gets clicked more because it makes them more revenue, if you get clicked 10% of the time for a keyword in position 2 and your competitor gets clicked 8% of the time, Google wants to show your ad and even gives you a discount of sorts on the click via higher quality score.)

So, if you google your keywords to see how your ad appears, you have 2 choices. Click on your ad, which will cost you money or dont click and lower your click through rate which can lower your quality score, especially if you do it often.

The issue of polluting data is that now your agency or whoever is managing your SEM sees either a lower click through rate if your dont click or a lower conversion rate if you do click and will make optimization decisions without knowing how many searches were you.

Google has an ad preview tool in Adwords to let you check without impacting your data.

In the greater scheme of things, if its a high volume query, the harm is minimal, but if its a high value term with low search volume (i.e. a big ticket local service) it can really hurt you.

If you are interacting with your own ads, the data you get at the end (especially if you are low traffic/a small player) will not be representative of reality, as your actions would have hurt/polluted the result.