Ask HR, there should be a time charging document.
Some of the requirements regarding charging will be because of the software they use. Some limit you to the 0.25 hour others to the 0.10 hours. Others will be how they handle lunch, and is it mandatory.
Generally if you are supposed to keep track of the starting and ending time, keep the exact time, though you can drop the seconds. If you can only record the delta time you will always want to round the hours worked down. If you can charge in 6 minute increments (0.10 hours), if you record that you worked 8.1 hours you better not have worked for 8 hours and 5 minutes.
Sometimes hours are directly billed to a customer. Over billing can result in penalties to the company. It can also result in you being fired. I have known government agencies that pulled the turnstile logs for the building to prove that an employee or contractor didn't work 8 hours that day.
Sometimes you hours aren't directly billed to the customer but the company doesn't want to overpay you. Therefore they tend to keep the rules the same no matter how you are billed, just to minimize confusion.
There are many rules regarding charging. Some of the rules will be same for all employees, others will only apply based on if they are eligible for Overtime pay. Being full time doesn't mean the rules don't apply, since if you aren't at work they will want you to charge vacation or sick.
You should expect guidance from HR.