Well, when it comes to "Side Missions" in other GTA's, once the Side Mission or Race has been passed, if you replay it and then fail the Side Mission or Race, it would NOT be added into the Stats. However, there is a Wins and Losses Stat in IV, TLaD, and TBoGT for these Races that each GTA is given. I am not entirely 100% sure, but I think even if you pass all nine Bike Races in TLaD, for instance, if you fail one of them later on to get one of the "Rare" Bikes like the Double T Custom and Hakuchou Custom, I believe it's actually adding a "Loss" +1 to the Stats. Since you can literally get such Rare Vehicles at any time, even after 100%, it's just not worth it, at least in my case to get those. It's also not worth it in my case to get any of the Rare Vehicles from Gang Wars, either. Story Missions, Random Encounters, and Stubbs' Dirty Laundry are the main areas of the game where the true Special Vehicles come from. Rare means you can get them at any time, but just rare to get some of them. Special means that once that specific Story or Side Mission is passed, you can never get them again unless you start a new game. Because of this, despite some of the Gang War Vehicles especially being ultra Rare, I am NOT going to obtain them anymore and the same goes for TBoGT with Drug War Vehicles.
As for V, I got that "Solid Gold, Baby" Achievement. I literally didn't settle for Silver or Bronze on ANYTHING in the Game. I did all Story Missions with Gold and all Strangers and Freaks with Gold. I did all Strangers and Freaks, even though only 20 of them are required for 100%. I did all 66 of them, however, not all 66 count as a Mission for a Medal. The ones that do count, however, including "The Last One," I believe are 46 if I recall correctly. When I go to hit Pause and reload the Game, you can see the Mission Names, Medals, Percents that you have got and I literally have Gold on the entire Game. Same in CW, there is only some instances where I found Gold nearly impossible and just settled for Silver or even Bronze, but in V, I did ALL GOLD/100%. I even did all 50 Stunt Jumps, 50 Under the Bridges, 15 Knife Flights, Gold on Shooting Range for all 3 Characters, Gold on Flight School, etc, etc. But when it comes to TBoGT, I never cared to get 100% on the 26 Missions until now. Since I am doing TBoGT over with, I might as well get 100% on those Missions, too.
Before the past few days, I would of said yes, definitely, to keep the Gang Burrito, Slamvan, or Comet from a Gang War, but after today, my opinion has changed since, despite how incredibly RARE some of those are to find, even after HUNDREDS of Gang Wars have been completed, they can STILL be obtained at any time, if you get lucky to get one to spawn. Whereas anything from the Story, Random Encounters, and Stubbs' Dirty Laundry are one time only Special Vehicles which you can't get ever again unless you start a new game. For a MAIN Save, I'd say to get just Special Vehicles, but for a secondary or third Save, you should get Rare Vehicles from Gang Wars and such.
When it comes to TBoGT, since Missions can be replayed to get a lot of the Special Vehicles, it's best to get things that come from Random Encounters and Club Management Missions since these are things in which can NOT be repeated or replayed, unless you start a new game. So for TBoGT, those Special Vehicles are priority over everything else, in my case. For the extra space, you can get 6 Unique Vehicles, even though 5 of the 6 can be obtained anytime using a Replay, and for the remaining one or two spots that you have left, you can get the EC Bright Red Comet from Packie and the EC Green and Lustered Vincent from Packie, as well, and both are rather difficult and/or time consuming to get, despite both being obtainable at the end of the game or after 100%, using a Replay.
Ultimately, it's up to you what to get, but the priority is shifted to Special Vehicles in which can NOT be obtained ever again unless you start a new game. Gang War Vehicles and Race Vehicles in TLaD are rather pointless. Just like some of the Story Mission Special Vehicles and Drug War Vehicles in TBoGT are rather pointless. When it comes to III, VC, SA, LCS, VCS, IV, CW, and V, I have done this very thing and obtained everything I should of obtained, but when it comes to TLaD and TBoGT, I have quite a few RARE Vehicles that can be obtained any time and because of that, it's good I will be re-doing them.