Not a one that's completely dead, but say an AGM battery that's at 75% and I want to top it off before trying to use it again. Can I use a regular lead acid battery charger? Do I have to worry about if the charger has a desulfation circuit (which they ALL seem to now, I couldn't find one without, I know they're death to Li-Po batteries)?
You don't need to worry about the desulfation circuit, although there are mixed reviews about how effective they are.
However, AGMs are more sensitive to overcharging than a standard SLA (sealed lead-acid) batteries. Actually, it's not the main charging cycle that is harmful, but the float-charge cycle that is used to top them off. If you are getting a new charger, make sure it has a setting for AGM batteries.
More detailed information can be found here.