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 $173.90   8.577%   $188.8154   $188.82   $188.80   $188.80  
  Purchase #2 $144.93   11.598%   $161.739   $161.74   $161.75   $161.70  
  Purchase #3 $146.51   6.152%   $155.5233   $155.52   $155.50   $155.50  
  Purchase #4 $96.58   4.250%   $100.6847   $100.68   $100.70   $100.70  
  Purchase #5 $31.49   3.197%   $32.4967   $32.50   $32.50   $32.50  
  Purchase #6 $222.43   12.115%   $249.3774   $249.38   $249.40   $249.40  
  Purchase #7 $24.83   9.562%   $27.2042   $27.20   $27.20   $27.20  
  7 purchase totals = $840.67       $915.8407   $915.84   $915.85   $915.80