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 $21.30   10.319%   $23.4979   $23.50   $23.50   $23.50  
  Purchase #2 $8.40   5.203%   $8.8371   $8.84   $8.85   $8.80  
  Purchase #3 $173.38   9.785%   $190.3452   $190.35   $190.35   $190.30  
  Purchase #4 $109.48   3.215%   $112.9998   $113.00   $113.00   $113.00  
  Purchase #5 $168.87   11.774%   $188.7528   $188.75   $188.75   $188.80  
  Purchase #6 $93.94   9.427%   $102.7957   $102.80   $102.80   $102.80  
  Purchase #7 $189.56   11.425%   $211.2172   $211.22   $211.20   $211.20  
  7 purchase totals = $764.93       $838.4457   $838.46   $838.45   $838.40