There is no reason not to do this.It gets better. I know they teach you to always do your deepest dives first but sometimes we will do a shallower dive to get comfortable, then go for the big dive of the day.
The "deepest dive first" sule is a relic of old advice related to shortening surface intervals between dives when using tables. It is not based on diving safety. Over time it went from a mere suggestion to a rule without anyone really understanding why, as was revealed in a workshop convened to figure out where the heck this rule came from. If your computer (or tables) tell you that you have had enough surface interval to do the dive you intend to do, it does not matter if it is deeper than a previous dive.