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 $242.54   11.757%   $271.0554   $271.06   $271.05   $271.10  
  Purchase #2 $110.50   8.496%   $119.8881   $119.89   $119.90   $119.90  
  Purchase #3 $90.43   8.107%   $97.7612   $97.76   $97.75   $97.80  
  Purchase #4 $206.85   12.467%   $232.638   $232.64   $232.65   $232.60  
  Purchase #5 $126.84   12.747%   $143.0083   $143.01   $143.00   $143.00  
  Purchase #6 $233.91   9.694%   $256.5852   $256.59   $256.60   $256.60  
  Purchase #7 $8.27   4.764%   $8.664   $8.66   $8.65   $8.70  
  7 purchase totals = $1,019.34       $1,129.6002   $1,129.61   $1,129.60   $1,129.70