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 $246.26   13.310%   $279.0372   $279.04   $279.05   $279.00  
  Purchase #2 $56.37   5.278%   $59.3452   $59.35   $59.35   $59.30  
  Purchase #3 $115.37   7.488%   $124.0089   $124.01   $124.00   $124.00  
  Purchase #4 $23.98   7.496%   $25.7775   $25.78   $25.80   $25.80  
  Purchase #5 $204.75   4.638%   $214.2463   $214.25   $214.25   $214.20  
  Purchase #6 $169.74   11.124%   $188.6219   $188.62   $188.60   $188.60  
  Purchase #7 $20.94   11.186%   $23.2823   $23.28   $23.30   $23.30  
  7 purchase totals = $837.41       $914.3193   $914.33   $914.35   $914.20