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 $28.41   8.643%   $30.8655   $30.87   $30.85   $30.90  
  Purchase #2 $202.59   3.873%   $210.4363   $210.44   $210.45   $210.40  
  Purchase #3 $180.71   13.534%   $205.1673   $205.17   $205.15   $205.20  
  Purchase #4 $55.83   11.387%   $62.1874   $62.19   $62.20   $62.20  
  Purchase #5 $25.93   12.294%   $29.1178   $29.12   $29.10   $29.10  
  Purchase #6 $55.98   4.829%   $58.6833   $58.68   $58.70   $58.70  
  Purchase #7 $79.90   10.676%   $88.4301   $88.43   $88.45   $88.40  
  7 purchase totals = $629.35       $684.8877   $684.90   $684.90   $684.90