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 $53.46   10.192%   $58.9086   $58.91   $58.90   $58.90  
  Purchase #2 $73.72   11.446%   $82.158   $82.16   $82.15   $82.20  
  Purchase #3 $72.70   8.512%   $78.8882   $78.89   $78.90   $78.90  
  Purchase #4 $195.42   8.104%   $211.2568   $211.26   $211.25   $211.30  
  Purchase #5 $30.73   7.308%   $32.9757   $32.98   $33.00   $33.00  
  Purchase #6 $8.64   3.398%   $8.9336   $8.93   $8.95   $8.90  
  Purchase #7 $31.54   11.137%   $35.0526   $35.05   $35.05   $35.10  
  7 purchase totals = $466.21       $508.1735   $508.18   $508.20   $508.30