it depends on the table and the number of rows.
Numbering rows allows a user in a large data set the ability to visually anchor themselves within the data. This is particularly useful when you have pagination. However; row number breaks down very quickly when the user sorts the results within the data set. The number then loose context and can actually add to overall user frustration.
I find that numbering rows is best used for infinite scrolling tables.
I honestly however stay away from them as they cover up a larger issue which is good table search, filtering and setting the user up for success when locating key information.