It depends if those apps are actively processing in the background. If they are not, there will be no impact on battery life. The majority of apps fit that criteria, even many apps that use data for their content, such as news apps. Usually they only update their content when you launch the app or manually refresh. If left running in the background they wouldn't be processing anything or accessing data. In fact, multi-tasking would probably improve battery life in this case as you wouldn't need to close/re-launch when switching tasks which would force it to fetch data uneccessarily.
is there a way to check what is running on my iphone 3GS? or do they automatically close when you press the home button? I read something about holding the top button then holding the bottom button to actually close the app, but hardly ever do it.