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 $138.93   12.651%   $156.506   $156.51   $156.50   $156.50  
  Purchase #2 $65.72   4.345%   $68.5755   $68.58   $68.60   $68.60  
  Purchase #3 $221.80   11.537%   $247.3891   $247.39   $247.40   $247.40  
  Purchase #4 $233.93   4.775%   $245.1002   $245.10   $245.10   $245.10  
  Purchase #5 $188.38   7.269%   $202.0733   $202.07   $202.05   $202.10  
  Purchase #6 $241.88   12.669%   $272.5238   $272.52   $272.50   $272.50  
  Purchase #7 $176.44   3.350%   $182.3507   $182.35   $182.35   $182.40  
  7 purchase totals = $1,267.08       $1,374.5186   $1,374.52   $1,374.50   $1,374.60