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 $143.50   12.252%   $161.0816   $161.08   $161.10   $161.10  
  Purchase #2 $32.81   5.661%   $34.6674   $34.67   $34.65   $34.70  
  Purchase #3 $87.90   6.783%   $93.8623   $93.86   $93.85   $93.90  
  Purchase #4 $243.64   12.867%   $274.9892   $274.99   $275.00   $275.00  
  Purchase #5 $111.69   8.798%   $121.5165   $121.52   $121.50   $121.50  
  Purchase #6 $191.97   9.505%   $210.2167   $210.22   $210.20   $210.20  
  Purchase #7 $106.99   8.364%   $115.9386   $115.94   $115.95   $115.90  
  7 purchase totals = $918.50       $1,012.2723   $1,012.28   $1,012.25   $1,012.30