What kind of teams distinguish whether someone should use a bulky volcarona set or an offensive volcarona set? Should one always be used over the other?
That would depend on whether or not Volcarona's team needs it to stick around or just attack everything in sight once Heatran goes down. There is no should when it comes to Volcarona if it's not a hyper offense team. If it is ho, then you definitely want offensive set but for other typs of team, it would come down to how much of an opening they can give offensive Volcarona and the player's own preference
do boots negate poison types' ability to get rid of tspikes?
No according to bulbapedia
If a grounded Poison-type Pokémon enters the field while holding this item, it will still cause Toxic Spikes to be removed.