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 $21.90   8.530%   $23.7681   $23.77   $23.75   $23.80  
  Purchase #2 $93.94   3.125%   $96.8756   $96.88   $96.90   $96.90  
  Purchase #3 $10.30   9.698%   $11.2989   $11.30   $11.30   $11.30  
  Purchase #4 $175.72   10.263%   $193.7541   $193.75   $193.75   $193.80  
  Purchase #5 $21.44   12.778%   $24.1796   $24.18   $24.20   $24.20  
  Purchase #6 $66.51   7.337%   $71.3898   $71.39   $71.40   $71.40  
  Purchase #7 $211.65   4.448%   $221.0642   $221.06   $221.05   $221.10  
  7 purchase totals = $601.46       $642.3303   $642.33   $642.35   $642.50