My startup is currently looking for a law firm to represent us. We have several offers/engagement letters already and have noticed that some say the bill is due in 30 days and other letters say that the bill is due immediately upon receipt. Will this make any difference? Are firms willing to negotiate? This isn't currently an issue for us as we can pay either way.
No difference. Basically that's for when they start to calculate and accrue late fees and interest in case you don't pay. In the US its not uncommon for the bills for professional services to be due upon representation, but it doesn't mean you need to pay it the same day. Paying within 30 days is fine, so unless you don't pay your bills it doesn't really matter.
Do check the delinquency and collection policies of each office, though, as terms vary and there's usually no law on the matter.
The reality is that not everyone pays their bill on time. Firms react in different ways, and there are a variety of unpleasant consquences for being delinquent on your legal bills (no surprise there).
The billing partner ideally should communicate well on this topic and be available to explain the firm's practices. Due in 30 days vs. on presentment may be very different depending whether there is a retainer involved and whether advance payment is required. An important aspect of this is your ability to review a bill before payment, and when legal serivces are terminated for lack of payment--especially for ongoing litigation.
What really matters is that you, the law firm, and the billing partner are all comfortable on how you pay your bill. This should appear in a written representation agreement.