Do I need to restart Apache if I modified an htaccess file?
No you don't. Make sure the htaccess is really loaded. Take a look at your access log, for example /var/log/apache/access.log
It's already been answered, however you don't need to reload as they are read each time a request is made.
This is why on high performance sites you should avoid use them as you are causing an extra I/O event per request. When you're working with tens of thousands of hits an hour it's just too much overhead.
Coops.