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 $18.41   8.819%   $20.0336   $20.03   $20.05   $20.00  
  Purchase #2 $131.33   5.484%   $138.5321   $138.53   $138.55   $138.50  
  Purchase #3 $25.41   3.335%   $26.2574   $26.26   $26.25   $26.30  
  Purchase #4 $135.26   12.255%   $151.8361   $151.84   $151.85   $151.80  
  Purchase #5 $105.35   10.554%   $116.4686   $116.47   $116.45   $116.50  
  Purchase #6 $81.50   9.372%   $89.1382   $89.14   $89.15   $89.10  
  Purchase #7 $124.73   5.714%   $131.8571   $131.86   $131.85   $131.90  
  7 purchase totals = $621.99       $674.1231   $674.13   $674.15   $674.10