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 $39.38   8.651%   $42.7868   $42.79   $42.80   $42.80  
  Purchase #2 $19.62   7.496%   $21.0907   $21.09   $21.10   $21.10  
  Purchase #3 $46.81   6.666%   $49.9304   $49.93   $49.95   $49.90  
  Purchase #4 $124.48   4.148%   $129.6434   $129.64   $129.65   $129.60  
  Purchase #5 $203.42   10.640%   $225.0639   $225.06   $225.05   $225.10  
  Purchase #6 $194.64   4.513%   $203.4241   $203.42   $203.40   $203.40  
  Purchase #7 $212.32   11.854%   $237.4884   $237.49   $237.50   $237.50  
  7 purchase totals = $840.67       $909.4277   $909.42   $909.45   $909.40