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 $27.37   5.639%   $28.9134   $28.91   $28.90   $28.90  
  Purchase #2 $212.66   4.176%   $221.5407   $221.54   $221.55   $221.50  
  Purchase #3 $198.56   10.266%   $218.9442   $218.94   $218.95   $218.90  
  Purchase #4 $21.59   10.182%   $23.7883   $23.79   $23.80   $23.80  
  Purchase #5 $113.44   4.384%   $118.4132   $118.41   $118.40   $118.40  
  Purchase #6 $20.78   6.281%   $22.0852   $22.09   $22.10   $22.10  
  Purchase #7 $78.51   11.448%   $87.4978   $87.50   $87.50   $87.50  
  7 purchase totals = $672.91       $721.1828   $721.18   $721.20   $721.10