I use only say 5 or 6 points to create the vol curve across strikes... spline interpolation makes a smooth vol curve, which then gives me all the iv's for each strike. There's no bias. It's just a fitting technique. You fit the curve to the market... Interest rate wise, like @Martinghoul says, you should actually use the debit/credit interest rates you as a company or person get from your bank/broker. But that means your pricing will probably not fit the market... so you use market conform rates.
@JackRab why do you bother fitting a spline? Unless you're calculating greeks by bumping or trying to trade relative value/market-make I don't see the need. Sorry if I missed answer to this elsewhere.
Because I like to have the ability to use my own pricing to make sense of certain relations between options... I just use an excel sheet to plot vols and price options, not needed for real time... but to look for strategies and manage them. That way I have a better view of the "what-if-scenarios", especially concerning vol shifts...