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 $101.65   11.362%   $113.1995   $113.20   $113.20   $113.20  
  Purchase #2 $48.83   5.692%   $51.6094   $51.61   $51.60   $51.60  
  Purchase #3 $123.32   6.723%   $131.6108   $131.61   $131.60   $131.60  
  Purchase #4 $66.68   12.857%   $75.253   $75.25   $75.25   $75.30  
  Purchase #5 $10.26   11.783%   $11.4689   $11.47   $11.45   $11.50  
  Purchase #6 $93.85   9.763%   $103.0126   $103.01   $103.00   $103.00  
  Purchase #7 $110.35   6.709%   $117.7534   $117.75   $117.75   $117.80  
  7 purchase totals = $554.94       $603.9076   $603.90   $603.85   $604.00