Not a good idea for power tubes (or ANY tube, but the mini's are less susceptible). The reason: seal stress. A tube's seal at the pins are designed so the metal and glass expand at the exact same rate. This is achieved when the whole unit warms up, as in normal operation. By applying solder heat to one point, you've made a point-of-expansion and until the joint cools to the same temp as the rest of the tube, air molecules are seeping in.You could tin the tube leads with solder to make them a little bit bigger, hence tighter.