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 $122.30   4.720%   $128.0726   $128.07   $128.05   $128.10  
  Purchase #2 $5.84   8.499%   $6.3363   $6.34   $6.35   $6.30  
  Purchase #3 $133.37   6.127%   $141.5416   $141.54   $141.55   $141.50  
  Purchase #4 $138.28   11.112%   $153.6457   $153.65   $153.65   $153.60  
  Purchase #5 $25.46   8.552%   $27.6373   $27.64   $27.65   $27.60  
  Purchase #6 $24.25   9.837%   $26.6355   $26.64   $26.65   $26.60  
  Purchase #7 $189.33   13.825%   $215.5049   $215.50   $215.50   $215.50  
  7 purchase totals = $638.83       $699.3739   $699.38   $699.40   $699.20