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 $25.54   13.178%   $28.9057   $28.91   $28.90   $28.90  
  Purchase #2 $111.87   5.331%   $117.8338   $117.83   $117.85   $117.80  
  Purchase #3 $232.54   3.259%   $240.1185   $240.12   $240.10   $240.10  
  Purchase #4 $118.35   7.306%   $126.9967   $127.00   $127.00   $127.00  
  Purchase #5 $232.96   11.406%   $259.5314   $259.53   $259.55   $259.50  
  Purchase #6 $98.98   6.280%   $105.1959   $105.20   $105.20   $105.20  
  Purchase #7 $152.32   9.826%   $167.287   $167.29   $167.30   $167.30  
  7 purchase totals = $972.56       $1,045.8690   $1,045.88   $1,045.90   $1,045.80