When you're laying down tile, wiggle is usually a bad word. One of the most important lessons to learn is that you always want to take care of any corrections and cleanup before the tile begins to dry. Once mastic (aka tile adhesive) is cured, it's hard as a rock (and that's the intention of course, since it's meant to be the glue that holds the tile to the floor), which makes it extremely difficult to clean up if you get it somewhere other than under the tile.
After the tile is glued and dry, grouting is the next step. Spacers can be easily swept up, but as I collected them from some of the joint lines, I noticed there were areas where the mastic had squished its way between the tiles, leaving very little space for the grout to stick when applied. If I'd left it this way, the grout might have a difficult time staying where I need it, and eventually find its way back out from between the tile (or never stick at all).
Some of that extra dried mastic needed to be removed, but would it be possible? I took a gamble and tried out my Porter-Cable oscillating tool again. For most of the lines, a grout blade worked really well. The blade was exactly the width of the spacers and made quick work of ridding the dried glue in the narrow lines.
Some of the tiles joined together in a T shape, which made it harder to get at the right angle with the blade. For these, I switched back to a metal/wood blade to make sure that I didn't risk hitting the tile and chipping. I was thrilled at how well this worked.
Granted, you would always still want to get it right the first time, and sawing into mastic isn't something you should rely on to clean up every mistake. But since I'm no expert, mistakes are inevitable. Knowing that I'm not going to ruin it if I don't get it 100% right is just the kind of reassurance I need to actually do the project in the first place. I may not want wiggle room on the tile itself, but wiggle room for my mistakes is always welcome!