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 $249.82   12.483%   $281.005   $281.01   $281.00   $281.00  
  Purchase #2 $14.54   9.649%   $15.943   $15.94   $15.95   $15.90  
  Purchase #3 $126.58   7.168%   $135.6533   $135.65   $135.65   $135.70  
  Purchase #4 $104.49   3.654%   $108.3081   $108.31   $108.30   $108.30  
  Purchase #5 $74.86   13.248%   $84.7775   $84.78   $84.80   $84.80  
  Purchase #6 $138.44   9.403%   $151.4575   $151.46   $151.45   $151.50  
  Purchase #7 $194.92   5.803%   $206.2312   $206.23   $206.25   $206.20  
  7 purchase totals = $903.65       $983.3756   $983.38   $983.40   $983.40