In a typical solve I only plan the cross for two reasons. The first is that the moves in the cross are remembered in my short term memory and I concentrate on doing them fast right in the beginning. Then the whole time I solve the cross I'm noticing all of the relevant corners that appear, so there really isn't all that much need--for me--to search for f2l pieces because it will actually slow my cross and then my f2l because I won't be looking at the corner when the cross is finished. On top of all that, it's just too much information for me to try to coordinate in my head.

The second reason is simply because I hate running out of time on my inspection and being forced to start the solve with a really crap cross.

Once you get faster your f2l will flow a lot more smoothly and the tracking of corners isn't really needed; Extended cross is another story, but that isn't always practical.

I average around 25 seconds so your question may not apply, but I hope this has helped anyway.