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 $9.95   4.435%   $10.3913   $10.39   $10.40   $10.40  
  Purchase #2 $22.96   11.861%   $25.6833   $25.68   $25.70   $25.70  
  Purchase #3 $227.98   10.761%   $252.5129   $252.51   $252.50   $252.50  
  Purchase #4 $48.77   6.697%   $52.0361   $52.04   $52.05   $52.00  
  Purchase #5 $188.84   4.739%   $197.7891   $197.79   $197.80   $197.80  
  Purchase #6 $112.97   9.662%   $123.8852   $123.89   $123.90   $123.90  
  Purchase #7 $28.64   10.289%   $31.5868   $31.59   $31.60   $31.60  
  7 purchase totals = $640.11       $693.8847   $693.89   $693.95   $693.90