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 $115.96   4.721%   $121.4345   $121.43   $121.45   $121.40  
  Purchase #2 $32.72   11.185%   $36.3797   $36.38   $36.40   $36.40  
  Purchase #3 $67.65   5.286%   $71.226   $71.23   $71.25   $71.20  
  Purchase #4 $85.44   6.387%   $90.8971   $90.90   $90.90   $90.90  
  Purchase #5 $216.25   5.627%   $228.4184   $228.42   $228.40   $228.40  
  Purchase #6 $35.46   7.182%   $38.0067   $38.01   $38.00   $38.00  
  Purchase #7 $241.43   10.285%   $266.2611   $266.26   $266.25   $266.30  
  7 purchase totals = $794.91       $852.6235   $852.63   $852.65   $852.60