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 $25.31   10.203%   $27.8924   $27.89   $27.90   $27.90  
  Purchase #2 $5.34   7.604%   $5.7461   $5.75   $5.75   $5.70  
  Purchase #3 $69.37   12.596%   $78.1078   $78.11   $78.10   $78.10  
  Purchase #4 $128.95   9.296%   $140.9372   $140.94   $140.95   $140.90  
  Purchase #5 $144.92   8.576%   $157.3483   $157.35   $157.35   $157.30  
  Purchase #6 $227.92   7.427%   $244.8476   $244.85   $244.85   $244.80  
  Purchase #7 $42.51   11.482%   $47.391   $47.39   $47.40   $47.40  
  7 purchase totals = $644.32       $702.2704   $702.28   $702.30   $702.10