It really depends (among other things) on the product you are storing, which part of the country the tank(s) are in, what other tanks and processes exist at your location and what operating permits are being used for your tank(s) and process(es).  For VOC storage tanks above a certain size, the US EPA requires the use of a control device when the vapor pressure of the stored liquid is within certain limits.  Floating roofs are one type of approved control device.  Fixed cone or dome roofs are not required if an "external" floating roof is used.  The owner evaluates the purchase and maintenance costs of each option to decide which to purchase.  See http://www.access.gpo.gov/nara/cfr/cfrhtml_00/Title_40/40cfr60_00.html for a the US EPA regulations.  Search for "VOC" for the relevant sections.  There are also different regulations for the storage of volatile carcinogenic liquids such as Benzene.  For information on the design of internal floating roofs, the best option is to contact a manufacturer, one of the largest is ALLENTECH.  Try davidr@allentech.com.  The API (American Petroleum Institute) and ILTA (Independant Liquid Terminal Association) are good industry groups for information and standards.