To me it make no sense paying shitloads of commishs and fees more just to hit a price that is already available on January through implied prices. Even with less size displayed on Jan, you would get it filled as you are actually hitting the combination of Dec and Dec/Jan with implied prices. Just do Jan First as it is less liquid. Moreover,you would still have to put margin for 250 CL at first...
Yes, always use the implieds when it comes to the energy markets. Even if you are offsetting months to arrive at a position, that slippage is miniscule compared to legging those months flat price. Every day I waste crossed contract months doing different calendar combinations to arrive at slightly better flys and condors. No biggie. Besides, if the fees are that big of a deal to you just lease a Nymex seat for $1300 per month and get member rates.