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 $58.90   12.115%   $66.0357   $66.04   $66.05   $66.00  
  Purchase #2 $65.66   11.599%   $73.2759   $73.28   $73.30   $73.30  
  Purchase #3 $101.31   11.523%   $112.984   $112.98   $113.00   $113.00  
  Purchase #4 $136.46   4.184%   $142.1695   $142.17   $142.15   $142.20  
  Purchase #5 $88.73   10.731%   $98.2516   $98.25   $98.25   $98.30  
  Purchase #6 $211.82   7.756%   $228.2488   $228.25   $228.25   $228.20  
  Purchase #7 $161.58   6.797%   $172.5626   $172.56   $172.55   $172.60  
  7 purchase totals = $824.46       $893.5281   $893.53   $893.55   $893.60