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 $27.57   4.467%   $28.8016   $28.80   $28.80   $28.80  
  Purchase #2 $90.61   10.208%   $99.8595   $99.86   $99.85   $99.90  
  Purchase #3 $40.53   4.260%   $42.2566   $42.26   $42.25   $42.30  
  Purchase #4 $121.83   3.420%   $125.9966   $126.00   $126.00   $126.00  
  Purchase #5 $37.47   9.169%   $40.9056   $40.91   $40.90   $40.90  
  Purchase #6 $126.91   7.266%   $136.1313   $136.13   $136.15   $136.10  
  Purchase #7 $57.99   3.731%   $60.1536   $60.15   $60.15   $60.20  
  7 purchase totals = $502.91       $534.1048   $534.11   $534.10   $534.20