It was unknown amongst the Allied armies in which direction Napoleon would march.
He might have chosen to march into Belgium, which in addition to cutting Wellington off from supply could have led the Belgians to rise in revolt against the Kingdom of the Netherlands, of which they had only lately been forced to become a part.
He might have marched on Prussia instead - and I may be wrong, but I believe the Prussian Army had not yet fully concentrated, and so Blucher covered the route between Napoleon and Prussia.
In any case, the Allies expected to have to concentrate to fight Napoleon, and their plan was to do so once they knew the axis of his advance.
In order to do so, they would have had to have better scouting - and Wellington would have needed greater confidence that Napoleon was not hooking past him at Mons, or at least earlier scouting reports.
This is where the battles of Ligny and Quatre Bras come in - the French attacked the Prussians at Ligny, sending them reeling back in retreat, and at Quatre-Bras prevented Wellington's troops from advancing in time to assist the Prussians at Ligny.
Having thus cleared the Prussians from his right (as he thought) Napoleon turned on Wellington at Waterloo. Wellington and Blucher (or possibly Blucher's staff, given the injuries he took at Ligny) were in communication throughout the 17th, and so Wellington stood at Waterloo well aware of the fact that Blucher was coming. He had no guarantee that Blucher would arrive in time, but if the Prussians had been forced into a general retreat rather than re-grouping at Wavre, Wellington would have continued retreating.
In other words, the course of events clearly shows that Wellington and Blucher intended to force Napoleon to fight them together, and that Napoleon attempted, with incomplete success, to drive them apart so that he could defeat them in detail. So yes, an earlier link-up between the Allied armies would have meant Napoleon's efforts failed earlier.