Not necessarily. Remember you can strand less than 3-99-5 at the end of a month, but not at the beginning. (for lurkers, if the last day of the prior bid month was a LC day, PBS will look back and enforce 3-99-5. But you CAN bid an X-day, or a Reg line, on the first, thus 'stranding' a lesser block at the end of the prior month. A common technique)

I also like to bid for 3-day blocks when bidding reserve whenever possible. The point of leaving the 31 then 30th at the end is that it will work its way in inverse order of denying days to try to meet the prior commands. If it can meet your command and give you 3 day blocks or less, like at the end of a bid month, the order of the off days won't matter anyway. I'm not looking at December, but often there will be a 'remainder' when the Res days are divided by 3 (this also works when days get prorated down for known absences like training, Vacation, Mil Leave, ALPA, etc. CQ and other 'courses'/training are great ways to 'park' a Res LC day(s) before and/or after, which can also be part of that 'remainder'). I learned the hard way to always put "1" as the low end of a start condition, which accommodates having a 'remainder' like this. Also, if you end up with one 4 day block, and there is still a 'remainder' day, it can park it at the end and still make the rest of it work, because the last day off you 'preferred' was the 31st, then 30th (or as appropriate for the bid month).