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 $19.90   3.325%   $20.5617   $20.56   $20.55   $20.60  
  Purchase #2 $172.64   9.436%   $188.9303   $188.93   $188.95   $188.90  
  Purchase #3 $236.34   11.352%   $263.1693   $263.17   $263.15   $263.20  
  Purchase #4 $110.65   13.740%   $125.8533   $125.85   $125.85   $125.90  
  Purchase #5 $54.98   9.406%   $60.1514   $60.15   $60.15   $60.20  
  Purchase #6 $110.38   6.538%   $117.5966   $117.60   $117.60   $117.60  
  Purchase #7 $124.41   8.266%   $134.6937   $134.69   $134.70   $134.70  
  7 purchase totals = $829.30       $910.9563   $910.95   $910.95   $911.10