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 $92.83   6.712%   $99.0607   $99.06   $99.05   $99.10  
  Purchase #2 $178.56   6.698%   $190.5199   $190.52   $190.50   $190.50  
  Purchase #3 $244.62   5.693%   $258.5462   $258.55   $258.55   $258.50  
  Purchase #4 $195.52   11.826%   $218.6422   $218.64   $218.65   $218.60  
  Purchase #5 $180.81   7.633%   $194.6112   $194.61   $194.60   $194.60  
  Purchase #6 $242.32   5.216%   $254.9594   $254.96   $254.95   $255.00  
  Purchase #7 $9.40   10.226%   $10.3612   $10.36   $10.35   $10.40  
  7 purchase totals = $1,144.06       $1,226.7008   $1,226.70   $1,226.65   $1,226.70