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 $122.66   11.677%   $136.983   $136.98   $137.00   $137.00  
  Purchase #2 $65.68   6.168%   $69.7311   $69.73   $69.75   $69.70  
  Purchase #3 $102.59   7.431%   $110.2135   $110.21   $110.20   $110.20  
  Purchase #4 $54.68   12.550%   $61.5423   $61.54   $61.55   $61.50  
  Purchase #5 $31.28   6.641%   $33.3573   $33.36   $33.35   $33.40  
  Purchase #6 $172.53   3.430%   $178.4478   $178.45   $178.45   $178.40  
  Purchase #7 $200.71   11.588%   $223.9683   $223.97   $223.95   $224.00  
  7 purchase totals = $750.13       $814.2433   $814.24   $814.25   $814.20