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 $54.47   8.215%   $58.9447   $58.94   $58.95   $58.90  
  Purchase #2 $61.63   11.851%   $68.9338   $68.93   $68.95   $68.90  
  Purchase #3 $98.83   12.151%   $110.8388   $110.84   $110.85   $110.80  
  Purchase #4 $203.46   13.828%   $231.5944   $231.59   $231.60   $231.60  
  Purchase #5 $62.92   5.367%   $66.2969   $66.30   $66.30   $66.30  
  Purchase #6 $99.60   9.774%   $109.3349   $109.33   $109.35   $109.30  
  Purchase #7 $25.76   13.352%   $29.1995   $29.20   $29.20   $29.20  
  7 purchase totals = $606.67       $675.1430   $675.13   $675.20   $675.00