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 $244.43   12.209%   $274.2725   $274.27   $274.25   $274.30  
  Purchase #2 $86.95   11.353%   $96.8214   $96.82   $96.80   $96.80  
  Purchase #3 $193.25   5.678%   $204.2227   $204.22   $204.20   $204.20  
  Purchase #4 $74.46   11.772%   $83.2254   $83.23   $83.25   $83.20  
  Purchase #5 $189.69   7.273%   $203.4862   $203.49   $203.50   $203.50  
  Purchase #6 $124.70   4.849%   $130.7467   $130.75   $130.75   $130.70  
  Purchase #7 $141.62   11.589%   $158.0323   $158.03   $158.05   $158.00  
  7 purchase totals = $1,055.10       $1,150.8072   $1,150.81   $1,150.80   $1,150.70