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 $25.48   9.410%   $27.8777   $27.88   $27.90   $27.90  
  Purchase #2 $99.74   12.725%   $112.4319   $112.43   $112.45   $112.40  
  Purchase #3 $100.51   6.693%   $107.2371   $107.24   $107.25   $107.20  
  Purchase #4 $203.25   4.266%   $211.9206   $211.92   $211.90   $211.90  
  Purchase #5 $39.30   5.164%   $41.3295   $41.33   $41.35   $41.30  
  Purchase #6 $45.63   13.348%   $51.7207   $51.72   $51.70   $51.70  
  Purchase #7 $45.42   13.777%   $51.6775   $51.68   $51.70   $51.70  
  7 purchase totals = $559.33       $604.1950   $604.20   $604.25   $604.10