Virginia Gov. Mills E. Godwin said yesterday he will not veto a $10 million appropriation for Metro construction in Northern Virginia, but his statement left uncertain whether any of the money will actually leave the state treasury in Richmond.

The governor said he has decided to leave the appropriation in the state's budget this year because of the strings the legislature tied to the item. In 1976 Godwin, to the dismay of Northern Virginia legislators, vetoed a similar appropriation.

This year the General Assembly reinserted the funds into the state's $7.4 billion biennial budget, but said the funds could be released only after the governor approved a revised plan for the overall costs of the Washington subway system.

Godwin can easily withhold approval of the funds or defer the action to whomever takes his position in January when his term expires, according to Senate Majority Leader Adelard L. Brault (D. Fairfax).

All three of the major candidates for governor -- Democrats Henry E. Howell and Andrew P. Miller and Republican John N. Dalton -- have indicated in campaign appearances that they favor the principle of state aid to Metro, provided a study of the system puts a lid on overall costs.

Godwin had been under intense pressure from the state's highway lobby to "stick to your guns" and veto the item again this year. Northern Virginians also trooped to his Richmond office urging approval of the item, which was one of eight bills his office had announced was being considered for a veto.

Godwin yesterday vetoed three of the 719 bills passed by the legislature, killing legislation that would have reduced the penalties for use of radar detection devices and two identical bills that would have prohibited prisoners on work-release plans from crossing picket lines to work.

Godwin said the bill that would have reduced the penalty for use of a radar detection device from a misdeameanor to a traffic infraction would have encouraged use of the devices. They "have only one purpose and that is to assist the driver in violating the speed laws of the state," Godwin said.

The prisoner-picket line bills were prompted by a strike at a meat packing plant in which work-release prisoners were trucked by state prison officials across a picket line. Some legislators said that the prisoners could be easily injured during a strike and that the state should not seem to take sides in labor disputes by allowing the prisoners to continue to work.

But Godwin disagreed sharply, saying that the bills, a House and Senate version of the same legislation, would not prejudice the status of the pris"same rights" allowed other employees, the right to cross a picket line.

The State Department of Corrections leaves up to a prisoner on work release whether he crosses a picket line, Godwin said. This policy favors neither employer or union and does not prejudice the status of the prisoner, he said.

Godwin has until midnight Sunday to sign bills passed by the 1977 General Assembly or they are automatically vetoed.His office the signing of 605 and announced the veto of six bills. Godwin aides have said that more vetoes are likely.

In approving the Metro item yesterday, Godwin said he was happy with the language that ties release of the funds to approval of a plan for financing the remainder of the system. "This provision speaks to the concerns I have repeatedly expressed over the escalating cost of Metro and (the transportation agency's) possible future requests for state funds," Godwin said in a prepared statement.

Although his office had drafted the actual language of the Metro appropriation for Sen. Brault, the senator said yesterday he had "some doubt" until he met with Godwin Tuesday that the appropriation would be approved.

At the meeting, Godwin, who had refused to commit himself to the appropriation, told Brault that "He hoped that after we got this money, we would insist on a cap on expenditures." Brault said he left the Richmond meeting "fairly secure" that the item would be approved.

If the funds, part of a mass transit appropriation within the highway department's construction budget are not released, the money will revert to the highway construction fund to be used for highways.

Earlier yesterday, Godwin signed another piece of legislation sought by Northern Virginia legislators alarmed over the rapid growth and high dollar volume of nightly bingo games in the regions. The legislation, introduced by Sen. Charles JJ. Colgan (D-Prince William), was prompted by what he said were abuses of the state's old bingo law by the Annandale Boys Club of Fairfax County.

Under the new law, bingo games will be limited to no more than three days a week except during carnivals. Sponsoring organizations must file financial reports with the local boards and they are barred from sharing any of the profits from the games with the owners of the premises of the games.