Yes, you may delete any files you are not currently using. These files are created whenever you test a survey locally. SSI Web copies some of the study's files and folders into a C:\Users\[Username]\AppData\Roaming\Sawtooth Software, Inc\SSI Web\LocalWeb\htdocs folder where the local Apache webserver may access them. This is also where any manually-created or data-generator data are stored prior to accumulation. So if you do not need the data, you may delete the files and folders within the htdocs folder but not the htdocs folder itself. If the htdocs folder is deleted it will be recreated the next time SSI Web is started. To find this folder in SSI Web 8, go to Test > Test Settings > Explore the Test Directory.
As long as you're cleaning up things, if you have run the CBC/HB standalone program, we also store some temporary files in the local Windows temp folder. This folder is found here: C:\Users\[Username]\AppData\Local\Temp. (If you do not see the AppData folder, you will need to enable Windows Explorer to view hidden items.). Most Sawtooth Software files look like #######.tmp. If you are not currently running CBC/HB, you may delete old Sawtooth Software files out that temp directory whenever you want.
This particular Temp folder was designed by Microsoft to be a temporary holding spot for files that are used to build applications. If a program needs a file to execute it will normally recreate it. Of course, some sloppy programmers from other companies may place files in there that are critical, but that should be the rare exception. By definition, files in the Temp folder should be disposable. By the way, if you run the Disk Cleanup tool in Windows and select the Temporary files checkbox, it will remove the files from this directory as well as others. To run the Disk Cleanup program, go to your Start/Run menu and type "cleanup".
Another candidate for the chopping block is the covariance file (covarniances.csv) in your CBC/HB ouput folder if you don't intend to run HB again using restart. Often this file is huge and takes up significantly more space than the rest of the CBC/HB output combined. Same goes for draws if they were inadvertently saved and you do not intend to use them.