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 $70.30   5.649%   $74.2712   $74.27   $74.25   $74.30  
  Purchase #2 $48.63   7.131%   $52.0978   $52.10   $52.10   $52.10  
  Purchase #3 $21.59   13.336%   $24.4692   $24.47   $24.45   $24.50  
  Purchase #4 $90.26   6.728%   $96.3327   $96.33   $96.35   $96.30  
  Purchase #5 $23.71   3.646%   $24.5745   $24.57   $24.55   $24.60  
  Purchase #6 $130.92   11.507%   $145.985   $145.99   $146.00   $146.00  
  Purchase #7 $117.28   4.106%   $122.0955   $122.10   $122.10   $122.10  
  7 purchase totals = $502.69       $539.8259   $539.83   $539.80   $539.90