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 $26.37   5.864%   $27.9163   $27.92   $27.90   $27.90  
  Purchase #2 $38.80   7.712%   $41.7923   $41.79   $41.80   $41.80  
  Purchase #3 $179.25   6.816%   $191.4677   $191.47   $191.45   $191.50  
  Purchase #4 $22.90   11.796%   $25.6013   $25.60   $25.60   $25.60  
  Purchase #5 $116.26   12.577%   $130.882   $130.88   $130.90   $130.90  
  Purchase #6 $120.93   3.299%   $124.9195   $124.92   $124.90   $124.90  
  Purchase #7 $81.37   13.453%   $92.3167   $92.32   $92.30   $92.30  
  7 purchase totals = $585.88       $634.8958   $634.90   $634.85   $634.90