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 $79.28   13.574%   $90.0415   $90.04   $90.05   $90.00  
  Purchase #2 $156.27   7.830%   $168.5059   $168.51   $168.50   $168.50  
  Purchase #3 $44.57   13.860%   $50.7474   $50.75   $50.75   $50.70  
  Purchase #4 $114.84   12.473%   $129.164   $129.16   $129.15   $129.20  
  Purchase #5 $232.95   11.406%   $259.5203   $259.52   $259.50   $259.50  
  Purchase #6 $182.79   13.537%   $207.5343   $207.53   $207.55   $207.50  
  Purchase #7 $14.51   9.455%   $15.8819   $15.88   $15.90   $15.90  
  7 purchase totals = $825.21       $921.3953   $921.39   $921.40   $921.30