Since I am saving hard for a home, should I claim more deductions on my regular paycheck in 2018 so I have more to pay towards down payment? And then use my mortgage to offset some taxes when I file in April 2019?
For many people mortgage interest and property taxes are what kicks them over the standard deduction and makes itemizing worthwhile. If you buy at the end of 2018, you'll have little to no interest to deduct unless you buy points (a separate discussion, but plenty of good articles about whether or not it makes sense to buy points). Likely you won't benefit from itemizing on your 2018 return, and therefore I wouldn't recommend adjusting your withholding unless you're consistently getting large refunds.
If you change your withholding too much, you could face underpayment penalties, since we pay income tax as we earn in the US. You wouldn't want to withhold so little that you didn't hit one of these criteria for avoiding underpayment penalties, per the IRS:
Generally, most taxpayers will avoid this penalty if they either owe
less than $1,000 in tax after subtracting their withholding and
estimated tax payments, or if they paid at least 90% of the tax for
the current year or 100% of the tax shown on the return for the prior
year, whichever is smaller.
There are other situations that would allow you to avoid the penalty as well, the above criteria are relevant in most cases.