A few versions back we got burned when SVN changed the format of the entries file, so we don't read that directly anymore as a "shortcut". In SlickEdit 2007 and later, the command line client is used for all operations. As long as the svn.exe commandline arguments (and the console output format) haven't changed, I don't see any problems. Looking over the SVN website, I don't see anything under their "breaking changes" section that affect console-based input/output.
I say give it a shot. (Easy for me to say, right?) At least there's a get-out-of-jail-free card in the form of a perl or python script that you can run to revert an upgraded working copy back to 1.4.x format.
Ironically, in the latest version of the Tools product, we moved away from the shell-and-scrape approach in favor of using the svn 1.4.6 libraries. It seemed to be the more stable approach. (And it is in fact much faster and more reliable than shell-and-scrape) However, there are binary incompatibilities between the 1.4.6 and 1.5.x libraries, so you can't update and older 1.4.6-based client by dropping in new DLLs. svn 1.5.0 went gold the same week we released Tools 2.0.2. Grrrr.