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 $91.88   12.123%   $103.0186   $103.02   $103.00   $103.00  
  Purchase #2 $115.74   6.214%   $122.9321   $122.93   $122.95   $122.90  
  Purchase #3 $215.44   7.552%   $231.71   $231.71   $231.70   $231.70  
  Purchase #4 $249.25   6.688%   $265.9198   $265.92   $265.90   $265.90  
  Purchase #5 $68.73   12.178%   $77.0999   $77.10   $77.10   $77.10  
  Purchase #6 $247.77   6.807%   $264.6357   $264.64   $264.65   $264.60  
  Purchase #7 $220.60   10.769%   $244.3564   $244.36   $244.35   $244.40  
  7 purchase totals = $1,209.41       $1,309.6725   $1,309.68   $1,309.65   $1,309.60