Going through customs at the airport is an uncertain process. If you arrive on a flight with lots of tourists and if you conduct yourselves like tourists, chances are your bags won't be opened. If asked for a tip and you give some money, you will be waved right through. Before asking, the inspector may open a suitcase, so make sure the first suitcases you put on the table are the ones that are the least suspicious.
If your bags are inspected and you are carrying a lot of stuff, you may or may not be charged.
The matter of customs inspectors asking for tips has come up before on this board so I know there are posters who think it terrible to do anything that furthers this practice. But, for myself, if coming with a lot of stuff in my bags that I hope will pass without trouble, I am quite happy to give a tip and zip through customs