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 $11.47   7.135%   $12.2884   $12.29   $12.30   $12.30  
  Purchase #2 $27.48   10.642%   $30.4044   $30.40   $30.40   $30.40  
  Purchase #3 $116.33   11.128%   $129.2752   $129.28   $129.30   $129.30  
  Purchase #4 $152.78   10.315%   $168.5393   $168.54   $168.55   $168.50  
  Purchase #5 $79.47   11.284%   $88.4374   $88.44   $88.45   $88.40  
  Purchase #6 $172.77   9.457%   $189.1089   $189.11   $189.10   $189.10  
  Purchase #7 $52.76   5.404%   $55.6112   $55.61   $55.60   $55.60  
  7 purchase totals = $613.06       $673.6648   $673.67   $673.70   $673.60