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 $17.54   7.660%   $18.8836   $18.88   $18.90   $18.90  
  Purchase #2 $157.76   9.816%   $173.2457   $173.25   $173.25   $173.20  
  Purchase #3 $78.26   4.180%   $81.5313   $81.53   $81.55   $81.50  
  Purchase #4 $105.50   4.201%   $109.9321   $109.93   $109.95   $109.90  
  Purchase #5 $85.42   12.392%   $96.0052   $96.01   $96.00   $96.00  
  Purchase #6 $172.65   5.540%   $182.2148   $182.21   $182.20   $182.20  
  Purchase #7 $204.27   13.521%   $231.8893   $231.89   $231.90   $231.90  
  7 purchase totals = $821.40       $893.7020   $893.70   $893.75   $893.60