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 $63.82   3.479%   $66.0403   $66.04   $66.05   $66.00  
  Purchase #2 $129.65   10.695%   $143.5161   $143.52   $143.50   $143.50  
  Purchase #3 $140.66   9.538%   $154.0762   $154.08   $154.10   $154.10  
  Purchase #4 $246.54   8.529%   $267.5674   $267.57   $267.55   $267.60  
  Purchase #5 $206.85   12.846%   $233.422   $233.42   $233.40   $233.40  
  Purchase #6 $187.29   3.532%   $193.9051   $193.91   $193.90   $193.90  
  Purchase #7 $218.91   6.322%   $232.7495   $232.75   $232.75   $232.70  
  7 purchase totals = $1,193.72       $1,291.2766   $1,291.29   $1,291.25   $1,291.20