When dealing with thunderstorms it is best to stay clear of clouds, when you can see. Barring that, the object is to keep the airplane as dry as possible and to avoid areas where the onset of heavy rain is sudden. When using radar, an area where rain goes from light to heavy in a short distance is described as having a steep gradient. That is the signature of a thunderstorm. The picture of this is a bit better on airborne weather radar than on Nexrad because of better definition. On Nexrad, the age of the picture when received in the airplane was a big question when the equipment first came into use. I think there is general consensus that that is not a problem as long as the information is used conservatively.
A broken line of thunderstorms poses a particular en route question. Sure, there might be a gap that meets the five miles from precipitation avoidance standard for thunderstorms that are not billed as severe. But the atmospheric conditions that are spawning that broken line exist to some extent in the precipitation-free areas, so going through a gap in precipitation might yield a rough ride. The best ride would be if you could slip through clear of clouds.
Embedded thunderstorms used to be a big challenge, but Nexrad helps sort out the rainy clouds from the regular clouds. Because some rainfall rates show as heavy on Nexrad even though there is no thunderstorm, lightning detection gear or the downlink of lightning information is helpful in deciding what rain is okay to fly through and what is not okay. Certainly I have learned that, from flying with both Nexrad and airborne weather radar, and despite assurances that both offer the same picture, Nexrad paints a gloomier picture. I have flown through red on Nexrad, where I always avoid red on the ship's radar. Flying through red on Nexrad is not something to do lightly and the decision has to be made based on gradients and lightning data.
If flying toward a low or a front with associated thunderstorms, count on them to be more numerous the closer you fly to the front. A strong squall line is actually more likely to be 100 or so miles ahead of the surface position of a front.
The controllers show weather on their scopes and have available the Nexrad picture. They can be some help with information and should certainly be consulted if there is no on-board equipment. But with the availability of portable weather receivers, as from Garmin and Control Vision, which can be carried in any airplane, every serious IFR pilot should be flying with Nexrad and lightning information.
There are conditions, especially in the Midwest, where the IFR altitudes available to a normally aspirated airplane are terribly unfriendly on stormy days. At such times, many pilots elect to fly VFR, beneath the clouds, while dodging all rain shafts.
Staying out of rain, in or out of clouds, doesn't guarantee a smooth ride. If it's a stormy day, the air is likely disturbed well away from rain, so always be ready to keep the airplane going through some substantial turbulence. More often than not, when an airplane is lost in turbulence the first thing that happens is a loss of control. So, control is job one.
If the turbulence is strong, the airspeed is fluctuating wildly and the rain is hard, the airplane might indeed be in a thunderstorm. If this happens, the battle is not automatically lost, but some really good flying will be required if the risk is to be managed. Forget holding altitude. Concentrate on keeping the wings level and the pitch attitude level. Adjust power to keep the airspeed as close to maneuvering speed as possible.
The conventional wisdom is to keep going straight ahead. Two reasons for that: It's probably the quickest way out, and any intentional bank angle will put the airplane that much closer to a loss of roll control.
Having said all that I'll qualify some of it.
If you are flying toward the east and the area of weather is moving east, the farther you fly into it, the worse it will be. An early decision to turn away here might be the best. If you are headed west into the same storm, the worst part of it will be the first part, and if you get through that then straight ahead might indeed be best.
When the destination is near, the temptation to delve into weather is the greatest.
Any temptation to fly an approach with thunderstorms in the area has to be dealt with through an understanding of the nature of those thunderstorms. What are the bases? Some thunderstorms have relatively high bases. I once took a lightning strike in smooth air beneath such a storm. Such storms are sometimes found on the slope of a warm front with the cold air beneath mitigating the flow of air down and out of the storm.