You can walk into to any BJ bar and have a beer with no need to choose a lady even though you might be pressured to. You're only required to buy a drink which is the least you can do if you're going to take up space. And if you can't find someone you like you can always come back.
Because many establishments have different amounts of girls coming in at certain times and there really is no perfect time to go in. However, it's best not to go just when the doors have just opened. Because most of the girls are probably still sleeping in.
However, BJ bars are not a bad place to just have a beer and hang out. Most establishments charge reasonable rates for beers and hard liquor.
A bottle of Thai Singha beer can cost about 100THB and some places do have happy hours or other early specials for local draft beers for cheap.
So overall I recommend paying a visit to a BJ bar just to check off the list of things to do in Bangkok's red light districts.