Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $55.98   8.268%   $60.6084   $60.61   $60.60   $60.60  
  Purchase #2 $104.81   4.590%   $109.6208   $109.62   $109.60   $109.60  
  Purchase #3 $146.87   5.149%   $154.4323   $154.43   $154.45   $154.40  
  Purchase #4 $22.28   8.438%   $24.16   $24.16   $24.15   $24.20  
  Purchase #5 $107.52   6.373%   $114.3722   $114.37   $114.35   $114.40  
  Purchase #6 $89.63   7.542%   $96.3899   $96.39   $96.40   $96.40  
  Purchase #7 $54.80   11.273%   $60.9776   $60.98   $61.00   $61.00  
  7 purchase totals = $581.89       $620.5612   $620.56   $620.55   $620.60