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 $20.29   4.418%   $21.1864   $21.19   $21.20   $21.20  
  Purchase #2 $85.96   5.353%   $90.5614   $90.56   $90.55   $90.60  
  Purchase #3 $120.41   7.263%   $129.1554   $129.16   $129.15   $129.20  
  Purchase #4 $44.88   4.770%   $47.0208   $47.02   $47.00   $47.00  
  Purchase #5 $103.56   13.805%   $117.8565   $117.86   $117.85   $117.90  
  Purchase #6 $184.57   13.174%   $208.8853   $208.89   $208.90   $208.90  
  Purchase #7 $243.54   11.246%   $270.9285   $270.93   $270.95   $270.90  
  7 purchase totals = $803.21       $885.5943   $885.61   $885.60   $885.70