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 $19.32   3.394%   $19.9757   $19.98   $20.00   $20.00  
  Purchase #2 $221.78   3.330%   $229.1653   $229.17   $229.15   $229.20  
  Purchase #3 $122.76   13.544%   $139.3866   $139.39   $139.40   $139.40  
  Purchase #4 $31.55   6.109%   $33.4774   $33.48   $33.50   $33.50  
  Purchase #5 $17.84   4.493%   $18.6416   $18.64   $18.65   $18.60  
  Purchase #6 $90.58   4.772%   $94.9025   $94.90   $94.90   $94.90  
  Purchase #7 $25.65   7.464%   $27.5645   $27.56   $27.55   $27.60  
  7 purchase totals = $529.48       $563.1136   $563.12   $563.15   $563.20