Personally I would wait until there were actual symptoms of WDs (if any) before using naltrexone (may not need it).
"Do not use any opiates for at least 7 days before starting naltrexone. You may need to stop certain opiate drugs (such as methadone) 10 to 14 days before starting naltrexone."
Above taken from here (webmd). Lots of info and possibly worth the read.
Edit: Great question... I learned something and it helps me grow. Thanks for it.