Hi gnrd,
As TRae mentioned, locations surrounding the foot of the bridge, or directly on top the bridge itself would not require you to show up early. I tend to watch the displays on De Lorimier and at times on Notre-Dame, and I can get to both streets relatively late without worrying about getting a good vantage point.
As well, as TRae again outlines, I, too, can't emphasize enough on how much weather conditions can impact attendance, and thus your chances of getting a good view on Notre-Dame - this is in part why I was successful in getting a nice view on that street last night despite getting there at just barely 5 minutes in advance. This is comparable to last week Saturday's filled up spots in the same area well ahead of time.
You may also want to consider De Lorimier as a choice as well since it allows you to come later and the fact that you can get a good view because of the street being on an incline relative to Notre-Dame - you'll easily see over everyone's heads around there as a result.
In short, you don't have to necessarily arrive early in these areas, unless of course that you're planning to go right up front along the steel fence on Notre-Dame when the weather is good. Although it is possible to still get a spot there once arriving late, it is very tedious and scarce, so it's best to get there at least between 8-8:45 p.m to secure a spot up front. The sidewalk there is also good since it, like De Lorimier, is on an incline, so you'd see over, though not quite to the same level like De Lorimier, everyone's heads.
In any case, providing you're coming by car, you'll be able to get free parking along De Lorimier, just so long as you get there by 8-8:30 p.m before they close off Rene-Levesque. If not by vehicle, then don't worry about that.
Trav.
