No, they don't need the force to block it.. but the stronger the force lighting, it can require more physical strength to be able to hold the lightsaber out against such powerful lightning.
In the book of "Revenge of the Sith", Mace' saber was being physically pushed back against his face. He was saying to Anakin that he was "toos strong" and the saber was being inched ever closer to his face. He was using Vapaad to send the lightning back into Palpatine, which palpatine seemed to not expect. It also says that Mace did get the advantage in the actual saber fight with Palpatine, something else that he didn't expect since he thought that he was so much more powerful than Mace and Yoda.
In the fight with Yoda, Palpatine grudgingly admitted to himself that Yoda's defense was perfect as he could find no holes in his technique. That is why it was Palpatine that stoped the lightsaber fight and started throwing the senate pods at Yoda, because Yoda was just too good at the Ataru style to keep up with the lightsaber battle.
When ObiWan's saber absorbed Dooku's lighting, it didn't require any extra strength to do so since Dooku was no where near as strong as Palpatine.
In the Star Wars universe, there are only 4 people known to have been able to deflect force lightniing, Palptaine, Dooku, Yoda and Luke. Up until Yoda did it, it was considered "Impossible" for a light side user to deflect force lightning. That is why when Yoda sent Dooku's lighting back to him.. Dooku was in complete Shock and had to quickly raise up his own hand to deflect his own lightning coming back to him. He was told that Yoda was old and weak and would fall under force lightning. He could scarely believe that Yoda was actually that strong to be able to stop it. Of course Dooku kept trying and eventually gave up once he saw that Yoda had no problem at all deflecting it.
The same is also true for their saber battle, as later on in their fight, Yoda's Ataru style was beginning to confuse Dooku and he knew that if this kept going much longer, he WAS going to lose. That is why he created the distraction with the stack that he pulled down towards Obi Wan and Anakin, so that he could use the distraction to escape and report these things to Palpatine.
Yoda of course had a far harder time with Palpatines lightning since he was much stronger than Dooku... Yoda was able to repulse it eventually..but it took all of his reserve power to finally accomplish it, which is why he didn't go back to continue trying to take Palpatine out after he fell from the force explosion.
In "Return of the Jedi" book, Luke was also able to repulse Palpatines force lightning for the first few salvo's, but then Palpatine got even more pissed off and amped up the power which eventually overcame even Luke's abilities and went into him. That is why Palpatine said to Luke "You're feeble skills are no match for the power of the dark side".
This shows you the great potential that the Skywalker line has, because Luke was not even an official Jedi yet, but he was already more powerful than most Jedi Masters most of whom can not deflect force lighting as only Yoda, as far as is known, was ever able to do so, then Luke after him. Sadly this short scene was never in the movies, but it SHOULD have been for sure.