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 $219.75   13.574%   $249.5789   $249.58   $249.60   $249.60  
  Purchase #2 $184.68   9.846%   $202.8636   $202.86   $202.85   $202.90  
  Purchase #3 $126.33   5.529%   $133.3148   $133.31   $133.30   $133.30  
  Purchase #4 $212.82   7.269%   $228.2899   $228.29   $228.30   $228.30  
  Purchase #5 $219.46   7.224%   $235.3138   $235.31   $235.30   $235.30  
  Purchase #6 $68.96   9.189%   $75.2967   $75.30   $75.30   $75.30  
  Purchase #7 $211.60   6.733%   $225.847   $225.85   $225.85   $225.80  
  7 purchase totals = $1,243.60       $1,350.5047   $1,350.50   $1,350.50   $1,350.50