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 $170.95   5.340%   $180.0787   $180.08   $180.10   $180.10  
  Purchase #2 $82.65   7.831%   $89.1223   $89.12   $89.10   $89.10  
  Purchase #3 $132.69   9.465%   $145.2491   $145.25   $145.25   $145.20  
  Purchase #4 $239.87   12.453%   $269.741   $269.74   $269.75   $269.70  
  Purchase #5 $71.25   8.693%   $77.4438   $77.44   $77.45   $77.40  
  Purchase #6 $62.90   3.670%   $65.2084   $65.21   $65.20   $65.20  
  Purchase #7 $6.29   3.709%   $6.5233   $6.52   $6.50   $6.50  
  7 purchase totals = $766.60       $833.3666   $833.36   $833.35   $833.20