Here's a compilation of all the regenerations:
In the first regeneration, Ben and Polly are crouched right over the Doctor as he changes. There is a wash of blinding light similar to the modern effect, but they aren't affected in any way by their proximity. Similarly, in the fourth regeneration, Adric, Nyssa, and Tegan are crouched right over the Doctor as he changes, although there's no burst of light in this version. And in the sixth regeneration, the Rani's hench-beast turns the Doctor over in mid-regeneration, to no evident ill effect.
Sarah Jane and the Brig are at a distance in the third regeneration, but there's no burst of energy there, just a simple fade. There does seem to be a burst of energy in the fifth regeneration, with Peri several feet away. The seventh is the first one that seems dangerous, with bursts of electricity around the body. Rose retreats when the ninth regeneration begins, but that could've been just alarm. The tenth is the most clearly violent one, with the energy tearing up the TARDIS interior.
Hmm. Looking at that, I wonder if you could make a case that the more energetic regenerations are the ones where the Doctor's resisting it the most. The First Doctor seemed resistant to change, and there was a burst of light, though not enough to affect the people next to him. The Second resisted fiercely, but we don't see the actual moment of transition. The Third was resigned to his fate and was unconscious, so there was no resistance and it went smoothly. Similarly with the Fourth, though he was conscious. The Fifth was more mentally troubled, as the montage reveals, and there was some outpouring of energy. The Sixth was knocked unconscious so there wasn't much energy, but some. The Seventh "died" in distress on the operating table, so while he was unconscious at the time it happened, maybe there was enough residual stress in his nervous system to cause a limited energy discharge.
But the Ninth seemed pretty resigned to it, so it's hard to explain the burst of energy there under this theory. Maybe it was just his general anger and edginess causing it. But the Tenth fought it fiercely and thus we got a violent burst of energy.
Ehh, not a perfect theory, but it makes things about as consistent as anything can be over the 50-year span of the franchise.