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 $13.49   11.865%   $15.0906   $15.09   $15.10   $15.10  
  Purchase #2 $55.32   3.335%   $57.1649   $57.16   $57.15   $57.20  
  Purchase #3 $48.83   9.236%   $53.3399   $53.34   $53.35   $53.30  
  Purchase #4 $203.92   12.135%   $228.6657   $228.67   $228.65   $228.70  
  Purchase #5 $139.94   11.549%   $156.1017   $156.10   $156.10   $156.10  
  Purchase #6 $107.91   11.363%   $120.1718   $120.17   $120.15   $120.20  
  Purchase #7 $64.27   8.721%   $69.875   $69.88   $69.90   $69.90  
  7 purchase totals = $633.68       $700.4096   $700.41   $700.40   $700.50