There are no tax consequences to rebalancing inside your tax-deferred accounts.
As to market timing, what I think you are looking for is the fabled rebalancing bonus. In fact, I just did this inside of my wife's 403(b). At the end of December 2018, I transferred about $7k from the bond fund into VINIX to get the total asset allocation corrected. YESTERDAY, I transferred about $8k from VINIX back into the bond fund, again to get the total allocation to my pre-selected set point. So for that little portion I bought around 216 and sold around 259, give or take.
So yes you do get little moments like this where you get a little benefit from rebalancing, but because the object is to maintain your desired asset allocation the adjustments are relatively small in comparison to your entire portfolio. IOW, you are moving a percentage point or two here and there, your are not going "all in" at the bottom and jumping out at just the right time at the top.