Dear Allycat,
A full treatment is the best option for healing. The relaxation part of Reiki can be "addictive". It is easy to focus on that especially as the client "knows" that you did something for them if they relax enough to go into sleep or an altered state. That is not necessarily indicative of a complete Reiki treatment. Many promote quiet during treatment to highten relaxation & it certainly does work & feels great. But Reiki is more than surface relaxation.
Many practitioners never do the back, its not unusual in some circles. Its not about the energy reaching the back from the front positions. If that was it, why not just do the crown & let it filter down to all the chakras?
If it is a worthwhile endeavor then it is worthwhile to do a complete treatment whenever possible. The key is what the client can handle, there are medical problems & disabilities that may prevent treating certain areas. So we get as close as we can to all the hand positions back & front & do what we can.
Half a treatment is better than none. But that should be the exception not the rule. IMHO, anyway. :)
P.S. When you have received Reiki from people whose mind was elsewhere, did slipshod work, etc. and compare that to a full treatment, you realize that sticking to the basics has benefits.
peace & joy,
holobon