On the other hand, you may well have investments in your taxable account with losses it which case you can sell those to put in your retirement accounts, which means not only will you get the benefit of moving "more" (in terms of number of shares) into your retirement accounts, but you'll also get to deduct the losses from your taxes next year. Losses will first offset any gains, and after the all gains are offset, up to $3000 of losses can offset ordinary income, which is even better since ordinary income is taxed at higher rates than long term gains.
If you have some losses, but selling the investments with losses won't raise as much as you want to move to retirement accounts then you could consider also selling some of the investments with gains in the right proportion so all the gains are offset with losses. This way you don't get an additional deduction, but at least you won't owe anything.
If all you have are investments with gains, then you might still want to sell some to move to retirement accounts, just realize that you'll owe taxes at your marginal capital gains rates. If you're in a low enough bracket this may be 0% although states often tax long term capital gains the same as regular income. Sell investments you've held more than one year so they're long term capital gains and sell the investments with the smallest percentage gains.
Make sure your taxable account is set to specific identification cost basis tracking so that you can sell specific tax lots to sell, otherwise you won't be able to be this precise with whether you're realizing gains or losses.