Since finance approvals are based on business history and overall credit, the timing of your application within the buying process should have little to no impact on your approval.
That said, I always recommend applying early in the buying process so you know what dollar amount you’ve been approved for. This allows you to shop for equipment with a general idea of your estimated payment amounts and also gives you plenty of time to think about variables like the number of months you can spread payments out over and the end-of-term purchase options. Having a pre-approval for the purchase may also help when negotiating the sales price as it will show you are serious about your purchase and have the ability to pay.
When applying, be sure to ask your lender or finance company how long your approval lasts. Applying too early or taking too long to make a purchase could cause your approval to expire, at which time you could be required to reapply.