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 $17.76   12.690%   $20.0137   $20.01   $20.00   $20.00  
  Purchase #2 $173.25   9.206%   $189.1994   $189.20   $189.20   $189.20  
  Purchase #3 $172.69   3.764%   $179.1901   $179.19   $179.20   $179.20  
  Purchase #4 $200.40   10.802%   $222.0472   $222.05   $222.05   $222.00  
  Purchase #5 $133.28   4.481%   $139.2523   $139.25   $139.25   $139.30  
  Purchase #6 $110.26   9.683%   $120.9365   $120.94   $120.95   $120.90  
  Purchase #7 $107.65   10.591%   $119.0512   $119.05   $119.05   $119.10  
  7 purchase totals = $915.29       $989.6904   $989.69   $989.70   $989.70