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 $6.48   4.605%   $6.7784   $6.78   $6.80   $6.80  
  Purchase #2 $96.29   3.737%   $99.8884   $99.89   $99.90   $99.90  
  Purchase #3 $159.41   4.867%   $167.1685   $167.17   $167.15   $167.20  
  Purchase #4 $181.52   13.747%   $206.4736   $206.47   $206.45   $206.50  
  Purchase #5 $89.43   7.484%   $96.1229   $96.12   $96.10   $96.10  
  Purchase #6 $34.47   7.119%   $36.9239   $36.92   $36.90   $36.90  
  Purchase #7 $21.67   4.635%   $22.6744   $22.67   $22.65   $22.70  
  7 purchase totals = $589.27       $636.0301   $636.02   $635.95   $636.10