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 $180.58   11.824%   $201.9318   $201.93   $201.95   $201.90  
  Purchase #2 $20.61   10.478%   $22.7695   $22.77   $22.75   $22.80  
  Purchase #3 $163.76   3.872%   $170.1008   $170.10   $170.10   $170.10  
  Purchase #4 $206.35   6.311%   $219.3727   $219.37   $219.35   $219.40  
  Purchase #5 $87.75   3.865%   $91.1415   $91.14   $91.15   $91.10  
  Purchase #6 $119.75   11.871%   $133.9655   $133.97   $133.95   $134.00  
  Purchase #7 $128.29   7.621%   $138.067   $138.07   $138.05   $138.10  
  7 purchase totals = $907.09       $977.3488   $977.35   $977.30   $977.40