If you read the drug information, the manufacturers of sublingual nitroglycerin tablets state that nitro is in fact contraindicated in the presence of an acute myocardial infarction. These people, who are, at least in theory, the experts on the drug, do not want it given in the setting of AMI While I do not have any studies in front of me, I believe that the evidence is increasingly showing that despite the theoretical benefits, there is little statistical benefit, if any.
Second, as to the theory of the reduction in preload - there is a reason that you should ALWAYS obtain a 12-lead prior to NTG administration. Early aspirin is very beneficial, however, early nitroglycerin can in fact be fatal. Should the patient be experiencing an AMI with RVI, then they are probably preload dependent - and reducing this can certainly cause death. This (and the training/education to detect aortic stenosis) are the reasons that BLS should never carry their own nitroglycerin - should they assist the pt with their own pre-prescribed NTG, much, if not all, of the liability can be shifted to the prescribing physician. On the other hand, if you give your own, you would not have a legal leg to stand on if the pt received NTG, had an adverse reaction, and had one of these two conditions. Even worse, if you are an ALS tech, and you didn't bother to check/perform these simple assessments.