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 $128.53   9.866%   $141.2108   $141.21   $141.20   $141.20  
  Purchase #2 $16.40   9.696%   $17.9901   $17.99   $18.00   $18.00  
  Purchase #3 $209.41   13.470%   $237.6175   $237.62   $237.60   $237.60  
  Purchase #4 $109.27   8.188%   $118.217   $118.22   $118.20   $118.20  
  Purchase #5 $5.80   9.663%   $6.3605   $6.36   $6.35   $6.40  
  Purchase #6 $7.79   10.446%   $8.6037   $8.60   $8.60   $8.60  
  Purchase #7 $126.46   7.163%   $135.5183   $135.52   $135.50   $135.50  
  7 purchase totals = $603.66       $665.5179   $665.52   $665.45   $665.50