What's your savings rate ($/month) and HELOC rate? If the first is high, and the second low, it might make sense to keep less than 50k liquid. Run the numbers if you have 25k liquid, and had to take the other 25k out of the HELOC.
25k invested long term could average 7%, which would net you 1750 in return after a year (before tax...assume your income falls in the 25% bracket, you'd pay 15% on the LTCG, or 262.50, netting 1487.50.)
If your savings rate was 5k/month (roughly 50% of the high end of the 25% tax bracket), you could pay of the HELOC in about 5 months. If your interest is 3.5% on the HELOC (which I've seen around here lately), you'd pay roughly $72/month in interest (25k * 3.5%/12), and over 5 months pay about $365 total. If you have enough deductions that the HELOC is tax deductible, your interest rate would fall to 2.6%, and the final bill would be $273.
The break-even rate of return of $365 on 25k is 1.4%, and $273 on 25k is 1.1%. If you think your investment could beat 1.4% or 1.1%, it would likely be worth it to invest 25k more than you had originally planned.