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 $241.83   5.874%   $256.0351   $256.04   $256.05   $256.00  
  Purchase #2 $59.63   3.644%   $61.8029   $61.80   $61.80   $61.80  
  Purchase #3 $248.27   12.333%   $278.8891   $278.89   $278.90   $278.90  
  Purchase #4 $187.78   3.620%   $194.5776   $194.58   $194.60   $194.60  
  Purchase #5 $45.77   11.224%   $50.9072   $50.91   $50.90   $50.90  
  Purchase #6 $177.30   13.467%   $201.177   $201.18   $201.20   $201.20  
  Purchase #7 $23.72   6.228%   $25.1973   $25.20   $25.20   $25.20  
  7 purchase totals = $984.30       $1,068.5862   $1,068.60   $1,068.65   $1,068.60