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 $9.41   9.800%   $10.3322   $10.33   $10.35   $10.30  
  Purchase #2 $41.95   6.696%   $44.759   $44.76   $44.75   $44.80  
  Purchase #3 $94.58   12.378%   $106.2871   $106.29   $106.30   $106.30  
  Purchase #4 $224.34   10.205%   $247.2339   $247.23   $247.25   $247.20  
  Purchase #5 $80.84   6.180%   $85.8359   $85.84   $85.85   $85.80  
  Purchase #6 $22.99   13.125%   $26.0074   $26.01   $26.00   $26.00  
  Purchase #7 $104.59   13.372%   $118.5758   $118.58   $118.60   $118.60  
  7 purchase totals = $578.70       $639.0313   $639.04   $639.10   $639.00