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 $9.66   3.388%   $9.9873   $9.99   $10.00   $10.00  
  Purchase #2 $128.81   8.304%   $139.5064   $139.51   $139.50   $139.50  
  Purchase #3 $183.47   4.387%   $191.5188   $191.52   $191.50   $191.50  
  Purchase #4 $241.97   6.748%   $258.2981   $258.30   $258.30   $258.30  
  Purchase #5 $241.99   11.582%   $270.0173   $270.02   $270.00   $270.00  
  Purchase #6 $196.56   12.370%   $220.8745   $220.87   $220.85   $220.90  
  Purchase #7 $23.74   3.844%   $24.6526   $24.65   $24.65   $24.70  
  7 purchase totals = $1,026.20       $1,114.8550   $1,114.86   $1,114.80   $1,114.90