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 $11.30   8.341%   $12.2425   $12.24   $12.25   $12.20  
  Purchase #2 $75.50   11.512%   $84.1916   $84.19   $84.20   $84.20  
  Purchase #3 $115.90   12.391%   $130.2612   $130.26   $130.25   $130.30  
  Purchase #4 $6.83   7.492%   $7.3417   $7.34   $7.35   $7.30  
  Purchase #5 $36.93   6.279%   $39.2488   $39.25   $39.25   $39.20  
  Purchase #6 $6.93   11.173%   $7.7043   $7.70   $7.70   $7.70  
  Purchase #7 $224.32   9.734%   $246.1553   $246.16   $246.15   $246.20  
  7 purchase totals = $477.71       $527.1454   $527.14   $527.15   $527.10