Hmm... it's not really that you stretch to a certain point, and stay there. A woman's sex life is not over once she has a baby, for example! (And most men's penises are significantly smaller than a baby....)
Also fingers are not necessarily more comfortable than penises what with nails and stuff.
So I don't think that sort of preparation is likely to have much correlation with how sex will feel for you once you get to that point.
It has a lot more to do with your general comfort level and arousal.
The vagina changes when you're aroused, from lubrication (VERY important) to actual size and depth.
So if you're comfortable with your boyfriend and very, very turned on, it's more likely to go well than if you're just trying to fit tab A into slot B, ya know?
And if you're not comfortable and it's hurting -- whether tab A is a penis or a finger -- stop, take it slow, give it more time.