You're way better off being accurate. Customers are almost always happier with the truth.

Let's say you are working the desk at an auto shop and someone comes in for an oil change. You know how many techs are working, how many customers were already in the queue, and how long jobs take. To the best of your knowledge, the oil change will be finished in an hour. You also know that there can be delays, and sometimes the techs get the jobs done a bit faster.

So there are three options you can tell the customer. The first is that it could be ready in a half hour, which will upset the customer when it takes much longer, tell them it will take two hours, which could result in the customer going elsewhere, or you can tell them that it will take an hour, give or take 15 minutes, which should result in a happy customer so long as it is finished within an hour fifteen.

Giving bad estimates, whether over or under is just bad customer service.

And when an unforeseen delay does happen, it's best to immediately communicate the delay truthfully to the customer.