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 $115.66   4.611%   $120.9931   $120.99   $121.00   $121.00  
  Purchase #2 $31.33   3.425%   $32.4031   $32.40   $32.40   $32.40  
  Purchase #3 $235.41   10.804%   $260.8437   $260.84   $260.85   $260.80  
  Purchase #4 $104.38   6.375%   $111.0342   $111.03   $111.05   $111.00  
  Purchase #5 $27.88   11.395%   $31.0569   $31.06   $31.05   $31.10  
  Purchase #6 $194.54   12.254%   $218.3789   $218.38   $218.40   $218.40  
  Purchase #7 $105.98   8.577%   $115.0699   $115.07   $115.05   $115.10  
  7 purchase totals = $815.18       $889.7798   $889.77   $889.80   $889.80