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 $26.39   6.397%   $28.0782   $28.08   $28.10   $28.10  
  Purchase #2 $39.65   3.259%   $40.9422   $40.94   $40.95   $40.90  
  Purchase #3 $201.26   10.202%   $221.7925   $221.79   $221.80   $221.80  
  Purchase #4 $105.33   3.648%   $109.1724   $109.17   $109.15   $109.20  
  Purchase #5 $207.33   12.645%   $233.5469   $233.55   $233.55   $233.50  
  Purchase #6 $18.28   9.594%   $20.0338   $20.03   $20.05   $20.00  
  Purchase #7 $225.87   10.437%   $249.4441   $249.44   $249.45   $249.40  
  7 purchase totals = $824.11       $903.0101   $903.00   $903.05   $902.90