a) you really don't want to be overhead as gliders may well be thermalling there and there may be winching
b) don't go near the glider circuit - it could ruin your entire day
c) joining from left base gives you no idea as to what is going on and is almost as bad as joining on a straight in approach
d) if you join downwind you are likely to be parallelling any glider in their circuit so you can get a good view of circuit traffic from glider and power side
e) if you are about to turn base at the same time as a glider, extend downwind or prepare for a go around and let him get on with his landing.
There was a nasty collision at Aston Down not so long ago with tug and glider turning into each other.
The Rules of the Air are quite clear - power gives way to gliders!