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 $18.89   10.403%   $20.8551   $20.86   $20.85   $20.90  
  Purchase #2 $103.36   12.830%   $116.6211   $116.62   $116.60   $116.60  
  Purchase #3 $167.68   6.186%   $178.0527   $178.05   $178.05   $178.10  
  Purchase #4 $216.79   8.613%   $235.4621   $235.46   $235.45   $235.50  
  Purchase #5 $12.71   4.524%   $13.285   $13.29   $13.30   $13.30  
  Purchase #6 $40.38   12.535%   $45.4416   $45.44   $45.45   $45.40  
  Purchase #7 $114.85   4.816%   $120.3812   $120.38   $120.40   $120.40  
  7 purchase totals = $674.66       $730.0988   $730.10   $730.10   $730.20