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 $133.66   3.646%   $138.5332   $138.53   $138.55   $138.50  
  Purchase #2 $15.92   9.218%   $17.3875   $17.39   $17.40   $17.40  
  Purchase #3 $50.43   3.683%   $52.2873   $52.29   $52.30   $52.30  
  Purchase #4 $9.51   4.631%   $9.9504   $9.95   $9.95   $10.00  
  Purchase #5 $245.56   7.264%   $263.3975   $263.40   $263.40   $263.40  
  Purchase #6 $34.45   9.496%   $37.7214   $37.72   $37.70   $37.70  
  Purchase #7 $113.64   9.442%   $124.3699   $124.37   $124.35   $124.40  
  7 purchase totals = $603.17       $643.6472   $643.65   $643.65   $643.70