A good point, although probably not too significant to someone selling OTM credit spreads. Your short calls would have to get pretty far in the money to be called away (which would be max loss for him).Quote from thegazelle:
If you trade SPY and you could face early exercise, especially with the dividends they pay.
The OP should also look at ES futures options. I've found getting fills at the midpoint is getting easier all the time. Simple verticals should fill pretty well.