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 $18.58   8.435%   $20.1472   $20.15   $20.15   $20.10  
  Purchase #2 $77.84   5.485%   $82.1095   $82.11   $82.10   $82.10  
  Purchase #3 $88.62   5.250%   $93.2726   $93.27   $93.25   $93.30  
  Purchase #4 $119.99   11.259%   $133.4997   $133.50   $133.50   $133.50  
  Purchase #5 $17.40   9.133%   $18.9891   $18.99   $19.00   $19.00  
  Purchase #6 $189.82   13.283%   $215.0338   $215.03   $215.05   $215.00  
  Purchase #7 $123.91   13.746%   $140.9427   $140.94   $140.95   $140.90  
  7 purchase totals = $636.16       $703.9946   $703.99   $704.00   $703.90