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 $53.32   12.646%   $60.0628   $60.06   $60.05   $60.10  
  Purchase #2 $244.59   11.601%   $272.9649   $272.96   $272.95   $273.00  
  Purchase #3 $26.93   12.449%   $30.2825   $30.28   $30.30   $30.30  
  Purchase #4 $177.46   12.239%   $199.1793   $199.18   $199.20   $199.20  
  Purchase #5 $130.87   11.841%   $146.3663   $146.37   $146.35   $146.40  
  Purchase #6 $168.46   9.780%   $184.9354   $184.94   $184.95   $184.90  
  Purchase #7 $20.56   10.747%   $22.7696   $22.77   $22.75   $22.80  
  7 purchase totals = $822.19       $916.5608   $916.56   $916.55   $916.70