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 $216.29   12.138%   $242.5433   $242.54   $242.55   $242.50  
  Purchase #2 $45.75   12.231%   $51.3457   $51.35   $51.35   $51.30  
  Purchase #3 $208.93   6.731%   $222.9931   $222.99   $223.00   $223.00  
  Purchase #4 $170.64   13.682%   $193.987   $193.99   $194.00   $194.00  
  Purchase #5 $135.25   8.361%   $146.5583   $146.56   $146.55   $146.60  
  Purchase #6 $122.47   10.265%   $135.0415   $135.04   $135.05   $135.00  
  Purchase #7 $93.72   5.412%   $98.7921   $98.79   $98.80   $98.80  
  7 purchase totals = $993.05       $1,091.2610   $1,091.26   $1,091.30   $1,091.20