CW
I think you will find that you are right to question what you'd heard. Flying in icing conditions is a serious matter and I think you specifically mentioned this in relation to piston twins. I have considerable experience of flying pistons for Public Transport purposes up and down the UK and into Northern Europe.
The first thing to consider is the equipment fit of the a/c. Is is equipped for icing conditions (boots, prop heat etc etc). If it is not then the simple answer is;
1. Don't go into icing conditions
2. If you find yourself in icing conditions get back out of them. (Consider 180 turn, descent below zero degree isotherm, diversion etc etc)
If the a/c is equipped for icing conditions then you may fly in icing up to (or should that be down to?) the 'stated' categorisation (ie equipped for 'moderate' icing). Simply, this means that if the de-ice/anti-ice equipment is managing the ice build up sufficiently then you are ok. If it is not you must follow the steps pointed out above for getting out of the problem; that's a command/experience decision. Your original question was about the 'procedures' a piston twin pilot should follow.
In the company where I worked flying C310,402 & 404's the procedures were pretty similar accross the whole fleet (all piston/unpressurised)and they were, upon entering icing conditions;
1. Prop heat on
2. Power and prop levers up to the 'top of the green' band
3. Operate the boots as soon as a build up of ice occurs on the leading edges (there used to be an idea of leaving the build up to 1/2-3/4 of an inch, but this theory has been superceded)
4. Monitor the situation and consider what options are available should the situation worsen.
From the above points, the first is self explainitory, but the second requires a bit more detail. Pushing the levers forward to the 'max continuous' settings does a number of things;
1. Increases prop RPM's and helps to 'fling' the ice off the props.
2. Increases TAS and lowers the nose slightly thus presenting less of the 'underside' to the icing. This was imperative on the C402 & C404 which were extremely prone to getting heavier as the ice built up, leading to the autopilot trimming the nose up to maintain the altitude, causing the speed to decay and presenting more of the underside to the relative airflow/icing leading to more build up of ice on areas that are not equipped with de-ice/ anti-ice and thus the situation getting worse and worse VERY QUICKLY.
3. The increased airspeed keeps you further away from the stall speed, which as stated will be incresing with ice build up.
So, what you heard is indeed VERY DANGEROUS in any aircraft; flying around at just above the stall is not something you want to be doing in crap weather conditions, think about what is going on in point 2 above and you can see that it is a sure recipe for disaster.
Also remember that flying piston twins generally means that you will be flying around 'in' the weather (especially if unpressurised - below FL100) so you are more likely to be exposed to icing and quite often you are being bounced and battered around at the same time.......it's not for the feint hearted, and contrary to what someone else said, piston twins do not generally have Airborne Wx Radar fitted. Some of our fleet did but they were the exception. It needs to be stressed that it comes more down to a command decision process rather than just a procedure. Keep your options open and really look at those wx charts and zero degree level in relation to MSA's before you go and look for the areas that might cause problems and think about enroute diversion airfields etc etc.
Otherwise it was always jolly fun half way accross the North Sea in the middle of the night in imbedded CB's with no wx radar and hail pounding the screen with your seat belt strapped as tight as you can get it and just a piece of urgent freight to keep you company!
Safe flying
PP