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 $8.37   6.159%   $8.8855   $8.89   $8.90   $8.90  
  Purchase #2 $19.72   10.537%   $21.7979   $21.80   $21.80   $21.80  
  Purchase #3 $218.38   13.415%   $247.6757   $247.68   $247.70   $247.70  
  Purchase #4 $92.44   11.385%   $102.9643   $102.96   $102.95   $103.00  
  Purchase #5 $20.95   8.720%   $22.7768   $22.78   $22.80   $22.80  
  Purchase #6 $210.78   8.233%   $228.1335   $228.13   $228.15   $228.10  
  Purchase #7 $83.84   7.173%   $89.8538   $89.85   $89.85   $89.90  
  7 purchase totals = $654.48       $722.0875   $722.09   $722.15   $722.20