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 $17.89   11.565%   $19.959   $19.96   $19.95   $20.00  
  Purchase #2 $118.59   9.459%   $129.8074   $129.81   $129.80   $129.80  
  Purchase #3 $93.61   10.413%   $103.3576   $103.36   $103.35   $103.40  
  Purchase #4 $75.84   6.867%   $81.0479   $81.05   $81.05   $81.00  
  Purchase #5 $75.95   7.579%   $81.7063   $81.71   $81.70   $81.70  
  Purchase #6 $87.47   4.463%   $91.3738   $91.37   $91.35   $91.40  
  Purchase #7 $163.30   9.412%   $178.6698   $178.67   $178.65   $178.70  
  7 purchase totals = $632.65       $685.9218   $685.93   $685.85   $686.00