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 $73.98   9.825%   $81.2485   $81.25   $81.25   $81.20  
  Purchase #2 $19.79   13.215%   $22.4052   $22.41   $22.40   $22.40  
  Purchase #3 $71.68   7.675%   $77.1814   $77.18   $77.20   $77.20  
  Purchase #4 $139.78   11.330%   $155.6171   $155.62   $155.60   $155.60  
  Purchase #5 $134.87   9.663%   $147.9025   $147.90   $147.90   $147.90  
  Purchase #6 $5.85   11.773%   $6.5387   $6.54   $6.55   $6.50  
  Purchase #7 $144.72   8.874%   $157.5625   $157.56   $157.55   $157.60  
  7 purchase totals = $590.67       $648.4559   $648.46   $648.45   $648.40