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 $123.99   3.325%   $128.1127   $128.11   $128.10   $128.10  
  Purchase #2 $13.83   9.456%   $15.1378   $15.14   $15.15   $15.10  
  Purchase #3 $184.41   3.844%   $191.4987   $191.50   $191.50   $191.50  
  Purchase #4 $146.32   7.638%   $157.4959   $157.50   $157.50   $157.50  
  Purchase #5 $246.78   4.548%   $258.0036   $258.00   $258.00   $258.00  
  Purchase #6 $32.38   5.829%   $34.2674   $34.27   $34.25   $34.30  
  Purchase #7 $36.29   13.769%   $41.2868   $41.29   $41.30   $41.30  
  7 purchase totals = $784.00       $825.8029   $825.81   $825.80   $825.80