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 $159.54   7.345%   $171.2582   $171.26   $171.25   $171.30  
  Purchase #2 $96.25   12.653%   $108.4285   $108.43   $108.45   $108.40  
  Purchase #3 $131.98   11.166%   $146.7169   $146.72   $146.70   $146.70  
  Purchase #4 $96.74   13.861%   $110.1491   $110.15   $110.15   $110.10  
  Purchase #5 $115.84   3.288%   $119.6488   $119.65   $119.65   $119.60  
  Purchase #6 $65.51   6.771%   $69.9457   $69.95   $69.95   $69.90  
  Purchase #7 $80.76   3.218%   $83.3589   $83.36   $83.35   $83.40  
  7 purchase totals = $746.62       $809.5061   $809.52   $809.50   $809.40