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 $55.29   6.821%   $59.0613   $59.06   $59.05   $59.10  
  Purchase #2 $34.97   13.505%   $39.6927   $39.69   $39.70   $39.70  
  Purchase #3 $170.76   6.747%   $182.2812   $182.28   $182.30   $182.30  
  Purchase #4 $228.72   6.497%   $243.5799   $243.58   $243.60   $243.60  
  Purchase #5 $231.70   10.860%   $256.8626   $256.86   $256.85   $256.90  
  Purchase #6 $160.63   5.434%   $169.3586   $169.36   $169.35   $169.40  
  Purchase #7 $44.91   9.312%   $49.092   $49.09   $49.10   $49.10  
  7 purchase totals = $926.98       $999.9283   $999.92   $999.95   $1,000.10