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 $28.99   3.263%   $29.9359   $29.94   $29.95   $29.90  
  Purchase #2 $58.63   12.768%   $66.1159   $66.12   $66.10   $66.10  
  Purchase #3 $138.65   13.402%   $157.2319   $157.23   $157.25   $157.20  
  Purchase #4 $160.90   11.447%   $179.3182   $179.32   $179.30   $179.30  
  Purchase #5 $29.75   4.479%   $31.0825   $31.08   $31.10   $31.10  
  Purchase #6 $83.37   7.821%   $89.8904   $89.89   $89.90   $89.90  
  Purchase #7 $164.27   8.165%   $177.6826   $177.68   $177.70   $177.70  
  7 purchase totals = $664.56       $731.2574   $731.26   $731.30   $731.20