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 $92.46   7.516%   $99.4093   $99.41   $99.40   $99.40  
  Purchase #2 $26.67   4.845%   $27.9622   $27.96   $27.95   $28.00  
  Purchase #3 $145.84   8.628%   $158.4231   $158.42   $158.40   $158.40  
  Purchase #4 $121.57   13.682%   $138.2032   $138.20   $138.20   $138.20  
  Purchase #5 $22.62   13.348%   $25.6393   $25.64   $25.65   $25.60  
  Purchase #6 $93.69   6.257%   $99.5522   $99.55   $99.55   $99.60  
  Purchase #7 $79.92   9.741%   $87.705   $87.71   $87.70   $87.70  
  7 purchase totals = $582.77       $636.8943   $636.89   $636.85   $636.90