In June, I plan to arrive in Rome on a Thursday. I need a Bor. Res. for the following Sunday. How far in advance do I need to make the reservation? can I wait until I get to Rome? Is morning or afternoon better for the Borghese?
If you are sure of the date, make it as soon as possible. 09:00 is a great time because a lot of people don't get up and out for the first tour. 17:00 is also nice IMHO. In other words, make it the first thing you do in the day. or the last before evening/dinner. This leaves you free to plan another adventure/site/activity for the day.