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 $250.98   13.851%   $285.7432   $285.74   $285.75   $285.70  
  Purchase #2 $110.51   4.618%   $115.6134   $115.61   $115.60   $115.60  
  Purchase #3 $67.69   10.398%   $74.7284   $74.73   $74.75   $74.70  
  Purchase #4 $52.37   6.813%   $55.938   $55.94   $55.95   $55.90  
  Purchase #5 $159.58   5.229%   $167.9244   $167.92   $167.90   $167.90  
  Purchase #6 $150.94   6.146%   $160.2168   $160.22   $160.20   $160.20  
  Purchase #7 $142.96   6.723%   $152.5712   $152.57   $152.55   $152.60  
  7 purchase totals = $935.03       $1,012.7354   $1,012.73   $1,012.70   $1,012.60