I get asked all the times if I have aluminum or titanium screws. Aluminum screws are very soft so they can't be used in any high stress areas, under the chassis or in any spots going into aluminum parts (where you need loctite).
As for Titanium screws, they are also weaker than alloy steel and will strip out very easily. They cost 10x more. You shouldn't use them under the chassis, in any aluminum parts that need to be loctite or any high stress areas. Titanium becomes very brittle under stress and the screws will sheer off under a hard impact. You shouldn't use them in any aluminum parts or under the aluminum chassis as there is a chemical reaction between the aluminum and titanium causing them to bind. When you go to take out the screw you'll strip the head out. Under the chassis the screws hex sockets also collect a lot of dirt which makes them that much more prone to stripping out since you may not clean out the hex socket well enough to get a good bite on the hex with your hex driver.
For the cost and the little weight saved you can spend the money to lose much more weight in other places: LiPo Rx packs, aftermarket aluminum hopups that are lighter and stronger than the factory alum parts, tires, foams (molded foams are much heavier than standard foams), lightened driveline parts, high grade aluminum pivot balls and ball end links, etc.
If you want to save weight but not sacrifice durability with your hardware, I suggest using Grade 12.9 alloy steel under the chassis, in any aluminum parts and any other high stress areas where the screw may take a hard impact. The alloy steel screws are not very expensive either. Then in low stress areas use the cheaper and lighter aluminum screws. They cost much less than Titanium screws and you can get them in cool anodized colors (red, blue, purple, gold and silver). You can also save a good amount of weight using aluminum locknuts. If you use the alloy screws and then aluminum screws/nuts like above you'll lose less weight, spend a lot LESS money and not sacrifice any strength. The places to use Titanium is in the tie rods since they don't typically experience loads or impacts where a high sheer strength is needed.
As for stainless steel screws, all I can say is they are good for boats, but the high stress an R/C car sees they are just not strong and high quality enough.
Best regards,