Dear friends,
In Iceland, you greet each other by rubbing nose...
In Most Western European countries, you greet by shaking hands...
In Arabic countries, you greet by hugging or kissing cheek to cheek...
In Indochinese countries, you greet by clapping your hands in prayer...
In Japan, you greet by bowing and since in Aikido you start the class by seiza (sitting posture) you do a rei i.e., a bowing from a sitting position.
I guess if you do want to you can shake hand with your sensei, if he doesn't mind...
Just my thought...
Boon