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 $244.98   11.699%   $273.6402   $273.64   $273.65   $273.60  
  Purchase #2 $109.75   7.106%   $117.5488   $117.55   $117.55   $117.50  
  Purchase #3 $24.84   12.557%   $27.9592   $27.96   $27.95   $28.00  
  Purchase #4 $230.87   12.827%   $260.4837   $260.48   $260.50   $260.50  
  Purchase #5 $92.54   13.788%   $105.2994   $105.30   $105.30   $105.30  
  Purchase #6 $37.33   7.536%   $40.1432   $40.14   $40.15   $40.10  
  Purchase #7 $215.89   4.802%   $226.257   $226.26   $226.25   $226.30  
  7 purchase totals = $956.20       $1,051.3315   $1,051.33   $1,051.35   $1,051.30