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 $130.80   6.178%   $138.8808   $138.88   $138.90   $138.90  
  Purchase #2 $35.84   12.577%   $40.3476   $40.35   $40.35   $40.30  
  Purchase #3 $186.96   8.599%   $203.0367   $203.04   $203.05   $203.00  
  Purchase #4 $50.29   10.445%   $55.5428   $55.54   $55.55   $55.50  
  Purchase #5 $19.50   3.146%   $20.1135   $20.11   $20.10   $20.10  
  Purchase #6 $219.31   13.775%   $249.52   $249.52   $249.50   $249.50  
  Purchase #7 $221.52   3.734%   $229.7916   $229.79   $229.80   $229.80  
  7 purchase totals = $864.22       $937.2330   $937.23   $937.25   $937.10