Personally I think you should build render times invisibly into your rate/ bids. An hourly rate for rendering time can get you into some problems with pricing integrity, among other things. For instance, an hourly charge for rendering rewards you for using slower machines to render on. Also, if you’re bidding against someone who has no line item on their bid for rendering time, you could be at a disadvantage both from the pricing perspective and from a perception perspective.
For similar reasons this is why I no not use an hourly charge for my services. It makes no sense in a creative business. If NBC calls you tomorrow and wants you to design them a new logo to replace the peacock, and you have your best design day ever and create something they love in 2 hours, are you going to send them a bill for $150 and call it a day? Hell no! Hourly rates reward people for being slow and often don’t take into account the value of creative or the value of what’s being delivered. You should be able to adjust your bid independently of an hourly rate taking into account who you’re working for and what you’re delivering. Hourly rates make more sense in an edit suite where you can expect a somewhat more consistent output of work per hour, but in design things are not so reliable.