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 $6.89   7.315%   $7.394   $7.39   $7.40   $7.40  
  Purchase #2 $233.39   3.705%   $242.0371   $242.04   $242.05   $242.00  
  Purchase #3 $139.88   12.833%   $157.8308   $157.83   $157.85   $157.80  
  Purchase #4 $24.69   4.432%   $25.7843   $25.78   $25.80   $25.80  
  Purchase #5 $134.26   7.561%   $144.4114   $144.41   $144.40   $144.40  
  Purchase #6 $64.87   5.684%   $68.5572   $68.56   $68.55   $68.60  
  Purchase #7 $191.40   10.468%   $211.4358   $211.44   $211.45   $211.40  
  7 purchase totals = $795.38       $857.4506   $857.45   $857.50   $857.40