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 $84.72   7.155%   $90.7817   $90.78   $90.80   $90.80  
  Purchase #2 $48.48   13.402%   $54.9773   $54.98   $55.00   $55.00  
  Purchase #3 $7.98   9.638%   $8.7491   $8.75   $8.75   $8.70  
  Purchase #4 $188.64   13.390%   $213.8989   $213.90   $213.90   $213.90  
  Purchase #5 $92.83   5.864%   $98.2736   $98.27   $98.25   $98.30  
  Purchase #6 $39.93   7.164%   $42.7906   $42.79   $42.80   $42.80  
  Purchase #7 $169.64   4.588%   $177.4231   $177.42   $177.40   $177.40  
  7 purchase totals = $632.22       $686.8943   $686.89   $686.90   $686.90