I would suggest keeping the progression fairly even. There is ample precedent for this in previous d20 games:
In D&D, XP awards are scaled so that each level requires, on average, the same number of encounters of a level that challenges a party of their level--and thus, theoretically, the same number of sessions or adventures. Under that system, I've noticed that PCs tend to reach 2nd level quickly, then settle into a fairly regular progression. Another GM I know, who has run at higher levels than I have, has observed that in his game, leveling slowed slightly after about 10th level, because a "typical" challenge could rapidly become a very deadly one at those levels.
Call of Cthuhu d20 uses a different system (with two options, actually), but is designed for an equivalent rate of advancement. Option 1 awards XP based on adventure goals (which are rarely "defeating monsters"); the XP received scales with character level. Option 2 is exactly what Blue Rose uses: no XP, characters level when the GM deems appropriate, and the suggested rate is every other adventure. (BTW, I'd heartily recommend option 1 for any GM who wants to quantify advancement in BR more precisely than the core rules do, or who wants a simplified XP system for D&D.)
In both games, having level progression remain at a steady rate means that players can expect to receive new abilities at a consistent, and fairly frequent, rate. And after all, gaining cool new abilities is the real heart of the reward system in any d20 game.
If you want to play around with that rate, go ahead--it's your game. But be sure to let your players know that you intend to do this--esp. if they've played other d20 games--so they will know what to expect.