Always paying a little less than your share adds up to a reputation.

Nobody notices any single instance, and that is exactly why it accumulates. People do not keep a number, they keep an impression, and impressions form out of moments nobody would bother mentioning. Being short on a given night is ordinary and everyone understands it. The pattern is what gets remembered, not the amount.

When money is tight, say so plainly rather than quietly rounding down. People are generous with a stated situation and unforgiving about a habit. On a night when you do have it, cover something small without being asked.