Usually, if you are careful enough, you can control the soil conditions, nutrients, water and lighting pretty precisely. However, the differences in the genetic makeup of the plant is totally up for a game of dice. It is when that little difference matters to you, that you should get an F1.
For example, if you're a championship show rose grower, you would probably care for that little bit. You'd want your roses to all bloom in time, at the same time and look pretty consistent too, which is pretty much guaranteed with an F1, if you don't bungle up the rest of the variables.
If you're growing plants commercially and you want them to all be ready before a certain time (e.g., pumpkins for Halloween and Thanksgiving in the US), you'd probably want to take the safer route and get F1s. However, if you're planning on continuing with them and propagating those plants, you should note that F2s are unstable. You probably already knew that. So that means getting F1s every year.
Normally, these are of no concern to the average gardener. Most people really couldn't care less if the package said that the roses were 4" in diameter and got ones that were only 3" or if their pumpkins matured a week or two late. But some might. If you fall in that group, you might want to get F1s.
Essentially, it all boils down to "Is it worth it to you?"