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 $7.99   10.392%   $8.8203   $8.82   $8.80   $8.80  
  Purchase #2 $160.65   7.303%   $172.3823   $172.38   $172.40   $172.40  
  Purchase #3 $50.67   10.542%   $56.0116   $56.01   $56.00   $56.00  
  Purchase #4 $94.45   11.413%   $105.2296   $105.23   $105.25   $105.20  
  Purchase #5 $129.83   7.817%   $139.9788   $139.98   $140.00   $140.00  
  Purchase #6 $122.50   12.857%   $138.2498   $138.25   $138.25   $138.20  
  Purchase #7 $106.45   13.847%   $121.1901   $121.19   $121.20   $121.20  
  7 purchase totals = $672.54       $741.8625   $741.86   $741.90   $741.80