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 $7.50   4.856%   $7.8642   $7.86   $7.85   $7.90  
  Purchase #2 $45.96   11.812%   $51.3888   $51.39   $51.40   $51.40  
  Purchase #3 $189.96   7.456%   $204.1234   $204.12   $204.10   $204.10  
  Purchase #4 $115.45   13.129%   $130.6074   $130.61   $130.60   $130.60  
  Purchase #5 $12.39   3.457%   $12.8183   $12.82   $12.80   $12.80  
  Purchase #6 $34.67   13.798%   $39.4538   $39.45   $39.45   $39.50  
  Purchase #7 $148.87   3.769%   $154.4809   $154.48   $154.50   $154.50  
  7 purchase totals = $554.80       $600.7368   $600.73   $600.70   $600.80