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 $124.61   6.276%   $132.4305   $132.43   $132.45   $132.40  
  Purchase #2 $223.27   9.674%   $244.8691   $244.87   $244.85   $244.90  
  Purchase #3 $104.33   13.464%   $118.377   $118.38   $118.40   $118.40  
  Purchase #4 $170.54   6.312%   $181.3045   $181.30   $181.30   $181.30  
  Purchase #5 $11.38   8.842%   $12.3862   $12.39   $12.40   $12.40  
  Purchase #6 $60.58   12.344%   $68.058   $68.06   $68.05   $68.10  
  Purchase #7 $200.80   13.794%   $228.4984   $228.50   $228.50   $228.50  
  7 purchase totals = $895.51       $985.9237   $985.93   $985.95   $986.00