Do in the manuscript as you intend to do in the final book. Your agent/editor needs to know exactly what you’ve got in mind in order to weigh the pros and cons with you in a useful way. And you will have that talk. Cursing may be absolutely right for your story, but it will also absolutely alienate some readers, so that discussion will be a part of the acquisition process. Using abbreviations is an option, but it won’t likely win you a wider audience because the people who would object to the real F-word will know it when they see it written as “effing”—and it’ll still rub them the wrong way. And then there’s the awkwardness factor. “F-ing” and “effing” can sound silly in a scene that shows a character mad or coarse enough to curse, which undercuts the effect of the swearing. If your character has to cuss, then let her cuss. Agents and editors know that some projects are edgier than others, and they take that into account when evaluating the cuss factor.