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 $159.87   3.280%   $165.1137   $165.11   $165.10   $165.10  
  Purchase #2 $96.67   4.414%   $100.937   $100.94   $100.95   $100.90  
  Purchase #3 $250.54   7.461%   $269.2328   $269.23   $269.25   $269.20  
  Purchase #4 $14.50   11.534%   $16.1724   $16.17   $16.15   $16.20  
  Purchase #5 $69.71   11.241%   $77.5461   $77.55   $77.55   $77.50  
  Purchase #6 $33.62   12.397%   $37.7879   $37.79   $37.80   $37.80  
  Purchase #7 $19.46   8.301%   $21.0754   $21.08   $21.10   $21.10  
  7 purchase totals = $644.37       $687.8653   $687.87   $687.90   $687.80