First of all, this is my personal opinion, and it is written from the experience of watching the race / riding the parcours several times, from the point of view of being a Belgian.
Most Belgians who watch the race do so without spending a night close to the parcours; it's an easy day trip from everyone living in the West of Belgium.
Some of the best places to watch are unfortunately also the most crowded, and this doesn't improve towards the end of the race. My personal favourites would be (all are curves, on secteurs paves):
If you want the full monty, go to the curve where Hushovd crashed:
https://goo.gl/maps/Qv77n1LjMRqiev759
Or anywhere along that stretch up until the Carrefour de L'arbre. But I can't promise you unobstructed views...! From around here:
https://goo.gl/maps/e71Lwo62Gt6iQibu6 to that curve, the cobbles are really bad and there is a great chance for some action. Gruson is not great because they go really fast there. If you want to have a good spot in the general area, I can recommend just standing somewhere here on this grassy triangle:
https://goo.gl/maps/oAA61XLvQTYfpdoXA. You can stand either at the East side to see them on the cobbles, or the west side:
https://goo.gl/maps/GHkr6THFwqLB7M8e8 . I know, that west side is asphalt, but that is the place they just took the curve from Carrefour, so you will see riders stretched out, tired, with low speed as they just took a close-to-180 degree curve, and it's full of mechanics with spare wheels and bikes in that place, so a good chance you will see riders stopping there (like WvA did last year) to fix their mechanical, you see this place exactly here:
View: https://youtu.be/8jnyZSxznCQ?t=7322
. Also note there is a big screen. I would rather fix my spot on the West side as the cobbles side will be very, very crowded with rows 4-5 wide.
But if you run fast, you can see them on the cobbles and on the asphalt (but most likely when you try this, you won't see them well at either side).
In any way, be there early enough if you don't want to walk some kms to the place where you want to stand, as parking is sometimes tricky and often improvised. Be aware that you can't park on the road in most places along those French roads, so you have to stick to the (sometimes soft) grassy shoulder, but if you already see a row of cars parked with 2 wheels on the road, just follow that principle.
Some other places, but most of those don't have big TV screens:
https://goo.gl/maps/KVLX8BZiDU752utM7 (Auchy-lez-Orchies) (half of the Sky team once crashed there)
https://goo.gl/maps/1KZnvK3N2zEMCHs78 (Mons-en-Pevele) (the famous curve where Cancellara started to go solo)
https://goo.gl/maps/2cA5v63TUgHFqnhY7 (Camphin-en-Pevele) (another hard curve)
I find most of the good stretches after Mons-en-Pevele either too busy (like Cysoing a Bourghelles, Camphin, Carrefour) or just not great for watching.
I would even rather watch at e.g. Warlaing a Brillon, Hornaing,... and watch the rest of the race on TV in a local pub of on my phone so I don't miss every little detail of the action (ofcourse you can watch the race later on TV and just focus on the atmosphere and the real live views).
If you fancy watching the riders and still have some comfort watching the race in a local pub (e.g. when it's cold and wet), you can try any pub in Merignies (e.g. here:
https://goo.gl/maps/quwqHw3YpkSB64bN8) or Orchies (more pubs). There aren't too many pubs in between Orchies and the last secteur in Hem.
One alternative for watching the race in a lazy way, is to go to the Velodrome. It's quite likely to find a place and you will see the espoir race finish as well + they ofcourse have a big screen.
With regards to hotels, as said, this is a day trip for me, so I would rather fancy a good hotel that's e.g. an hour away, than try to find a hotel on / very close to the parcours. Lille / Roubaix have plenty (but check the area, I would avoid some areas around Lille), but e.g. Tournai is a really nice, cosy city with a beautiful cathedral and as close (or even closer) to some secteurs than Lille / Roubaix. Brussels, Ghent, even Bruges are less than an hour away from Carrefour (the Belgian perspective ;-)