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 $25.49   9.861%   $28.0036   $28.00   $28.00   $28.00  
  Purchase #2 $49.30   10.713%   $54.5815   $54.58   $54.60   $54.60  
  Purchase #3 $138.62   4.755%   $145.2114   $145.21   $145.20   $145.20  
  Purchase #4 $156.80   10.353%   $173.0335   $173.03   $173.05   $173.00  
  Purchase #5 $164.85   3.362%   $170.3923   $170.39   $170.40   $170.40  
  Purchase #6 $14.93   3.568%   $15.4627   $15.46   $15.45   $15.50  
  Purchase #7 $133.45   7.848%   $143.9232   $143.92   $143.90   $143.90  
  7 purchase totals = $683.44       $730.6082   $730.59   $730.60   $730.60