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 $250.51   11.371%   $278.9955   $279.00   $279.00   $279.00  
  Purchase #2 $132.55   6.874%   $141.6615   $141.66   $141.65   $141.70  
  Purchase #3 $227.46   12.410%   $255.6878   $255.69   $255.70   $255.70  
  Purchase #4 $24.52   6.570%   $26.131   $26.13   $26.15   $26.10  
  Purchase #5 $35.29   5.640%   $37.2804   $37.28   $37.30   $37.30  
  Purchase #6 $46.27   8.836%   $50.3584   $50.36   $50.35   $50.40  
  Purchase #7 $238.75   8.267%   $258.4875   $258.49   $258.50   $258.50  
  7 purchase totals = $955.35       $1,048.6021   $1,048.61   $1,048.65   $1,048.70