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 $105.33   12.317%   $118.3035   $118.30   $118.30   $118.30  
  Purchase #2 $141.38   13.152%   $159.9743   $159.97   $159.95   $160.00  
  Purchase #3 $119.47   7.317%   $128.2116   $128.21   $128.20   $128.20  
  Purchase #4 $26.63   10.825%   $29.5127   $29.51   $29.50   $29.50  
  Purchase #5 $177.54   11.691%   $198.2962   $198.30   $198.30   $198.30  
  Purchase #6 $149.92   9.292%   $163.8506   $163.85   $163.85   $163.90  
  Purchase #7 $219.34   7.495%   $235.7795   $235.78   $235.80   $235.80  
  7 purchase totals = $939.61       $1,033.9284   $1,033.92   $1,033.90   $1,034.00