Insulin will always hinder fat loss when present, without exception. Circulating insulin literally closes up the fat cell, preventing it from releasing stored fat for use as energy. However, the question is not whether a single post-workout injection will inhibit fat loss, but rather, if the degree of inhibition is sufficient to completely undermine one's efforts during those times when insulin is not present.
I cannot provide a yes or no answer. The answer will depend on the type of insulin used, the amount of insulin used, how far along you are in your diet, whether you are ahead or behind schedule, and the effectiveness of your diet & PED program for initiating fat loss. For example, injecting 50 IU of Lantus at 3 weeks out will most certainly have a significant negative effect on your ability to drop that last 1-2% of bodyfat. On the other hand, if you are using Humalog at 12 weeks out, you should have no problem losing bodyfat easily.
Humalog is ideal when dieting, as it only stays active for 2-3 hours and when used post-workout, it is more likely to facilitate recovery than impair fat loss. No, you likely won't lose any bodyfat over those 2-3 hours of activity, but you will still have 21-22 hours of the day left to lose bodyfat. So, in this case, the benefits outweigh the negatives. You may be able to continue using Humalog post-workout all the way until the contest, but it will depend on how well you are progressing in your diet. If you are behind, you will probably want to eliminate it...and if you are on track, you will probably want to keep it in, as it will help maintain fullness and muscle tissue during the more physically stressfull parts of the diet.
I never recommend using long-acting insulins when dieting, at any time. Humulin R can be used for the first half of the diet if necessary, but Humalog is still preferable. However, most people can get away with using humulin post-workout for at least the first half of their diet...and sometimes longer, as long as they are still on track.