Should I shake bottles during conditioning?

I agree with your friend. I always shake my bottles first when I'm pouring for secondery fermentation. Be careful to make sure the first fementation is complete or you'll cause the bottle to explode because it's relesing to much CO2. It helps make a good head when pouring and helps the yeast to become active again especially if you're using carbonation drops that dont dissolve straight away.

I prefer to add sugar to the fermeter (after fermentation is completed and I'm ready for bottleing) instead to adding it to each individual bottle. I guess this way there's no need to shake the bottles...

I dont "shake" my bottles while theyre conditioning. After adding my priming sugar and filling my bottles I give them a few good turn overs, which in effect could be considered a light shaking, mostly to help disperse my priming sugars throughout the bottle. After that I put them away until its time to drink.

One reason to try and keep the yeast in suspension is that a better contact between yeast and beer helps the yeast to decrease diacetyl. Depending on whether you have had a proper secondary fermentation before bottling, this could be important. I agree however with the security-concerns already mentioned further up.

In general there is no reason to do this as it just prolongs the settling out of all the yeast. If you had good yeast health going into the bottling phase all should be fine.

I don't like to intervene with the process any more than necessary, unless something out of the ordinary occurs.

If your bottles don't carb up, or they seem inconsistantly carbonated, or you over chill the bottle due to poor storage prior to carbing; these are times when I'll try to rouse the yeast. But even then I wouldn't shake them. Upside down with gently swirling would be the best way.

A saftey note regarding glass and carbed bottles: You really have no idea if a glass bottle is carbed up until you open it. Its very conceivable that if you have some bottles that are flat and some that are carbed due to poor sugar distribution you might have some very highly carbed bottles sitting there along side flat ones. Starting to shake them because one suspects the yeast is just not doing its job may lead to a burst bottle. If a beer is overcarbed, and you agitate it, and rouse up all that wonderful nucleating yeast sediment in the bottle you could be create an explosive glass hand grenade. So use a towel and cover each bottle if you suspect that type of danger.

I wouldn't bother, but if you do, don't shake them hard, because it will denature some of the foam-producing proteins, and has a chance of reducing head retention.

Be lazy and let it do its thing.

I does help to shake the bottles when conditioning because it will help the yeast process the sugar faster and produce pressure in the head space that will eventually be the carbonation in your beer. You should only have to do this the first few days of conditioning.