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 $231.59   11.404%   $258.0005   $258.00   $258.00   $258.00  
  Purchase #2 $184.77   9.377%   $202.0959   $202.10   $202.10   $202.10  
  Purchase #3 $115.96   8.308%   $125.594   $125.59   $125.60   $125.60  
  Purchase #4 $42.48   11.145%   $47.2144   $47.21   $47.20   $47.20  
  Purchase #5 $240.87   6.594%   $256.753   $256.75   $256.75   $256.80  
  Purchase #6 $228.66   11.363%   $254.6426   $254.64   $254.65   $254.60  
  Purchase #7 $66.64   11.345%   $74.2003   $74.20   $74.20   $74.20  
  7 purchase totals = $1,110.97       $1,218.5007   $1,218.49   $1,218.50   $1,218.50